There is no need to be cautious in asking this question. For the reason the Nicene (here the Niceano Constanipolitan) creed here uses the wording, ὁμολογοῦμεν "ἓν βάρτισμα εἰς ἄφεσιν ἁμαρτιῶν" is that it is reflecting what the bible says.
Likewise, the reason the early church fathers include the word, ἓν is because of Paul's words: “εἷς κύριος, μία πίστις, ἓν βάπτισμα,” (Ἐφεσίους 4·5 THGNT-T)
Again, the reason these words are found in the creed is because they are found in the bible. And as such, they are 'fair game' for questions in this SE.
ⲉⲓⲥ is a fairly flexible preposition. While others include a listing from Wallace, I'm going to factor him out of this discussion since his listing for this word isn't extensive and since his bias (as a professor at DTS) gets in the way of his research. Let's, instead, look at the venerable BDF:
- Εἰς instead of ἐν in a local sense. In MGr εἰς has absorbed the related preposition ἐν (in conjunction with the disappearance of
the dative); in the NT ἐν appears almost twice as frequently as εἰς,
but the confusion of the two has begun in that εἰς often appears for
ἐν (ἐν for εἰς more rarely, §218). Cf. Hatzid. 210f.; Regard, Prép.
330–49. No NT writer except Mt is entirely free from the replacement
of ἐν by εἰς in a local sense, not even Lk in Acts where most of the
examples are found (Jn has the fewest); Mk 1:9 ἐβαπτίσθη εἰς τὸν
Ἰορδάνην (cf. Homil Clem 11.36.2 εἰς τὰς ... πηγάς; ἐν Mk 1:5,
Mt 3:6), Lk 11:7 εἰς τὴν κοίτην εἰσίν (ἐν D), A 8:40 εὑρέθη
εἰς Ἄζωτον, Jn 1:18 ὁ ὢν εἰς τὸν κόλπον (ἐκ τοῦ -ου syc)
τοῦ πατρός. The Epistles and, still more surprisingly, Rev exhibit a
correct differentiation between εἰς and ἐν in the local sense except
[p. 111] 1 P 5:12 (postscript in the author’s own hand) τὴν χάριν
... εἰς ἣν ἑοτήκατε (KLP; στῆτε ‘stand fast in it’ found in the
other MSS would suit εἰς, but according to the sense is very
unlikely). For 1 Jn 5:8 s. infra. Εἰς for ἐν is frequent in Hermas
(Humbert 74–6): Vis 1.2.2 ἔχουσα βιβλίον εἰς τὰς χεῖρας, 2.4.3,
Sim 1.2 etc.; s. also 2 Clem 8.2 (19.4?), Homil Clem 12.10. This εἰς
appears therefore to have been still a provincialism at the time;
however, the fact that some authors do not share the confusion is also
surprising. Examples from the LXX, Diodor. etc. in Jannaris §1548;
Vogeser 26ff.; Johannessohn ii 331f.; Mayser ii 2, 373; Humbert
passim; Oepke, TW ii 418f. n.
Additional Notes
Mk 1:39 κηρύσσων εἰς τὰς συναγωγάς (ἐν ταῖς συναγωγαῖς EF
al.), 2:1 εἰς οἶκόν ἐστιν AC al. (ἐν οἴκῳ SBDL), 10:10 (ἐν
AC al.), 13:3 καθημένου εἰς τὸ ὄρος (cf. Herm Sim 5.1.1; Musonius
43.18 Hense καθῆσθαι εἰς Σινώπην; correctly class. καθίζειν εἰς 2 Th 2:4), 13:9 εἰς συναγωγὰς δαρήσεσθε (= Mt 10:17 ἐν ταῖς
συναγωγαῖς [D εἰς τὰς σ.] αὐτῶν μαστιγώσουσιν ὑμᾶς), 13:16 ὁ
εἰς τὸν ἀγρόν (ἐν Mt 24:18, Lk 17:31).—Lk 4:23 γενόμενα
(‘happened’) εἰς τὴν (SB, εἰς DL, ἐν τῇ al.) Καφαρναούμ, 9:61,
21:37? A 2:5 εἰς Ἰερ. κατοικοῦντες (ἐν ScBCDE; like class. H 11:9
παρῴκησεν εἰς γῆν, Mt 2:23, 4:13; cf. Thuc. 2.102.6 κατοικισθεὶς
ἐς τοὺς ... τόπους), 2:27 OT (cf. 31) ἐγκαταλείψεις τὴν ψυχήν
μου εἰς ἅδην, 39 τοῖς εἰς μακράν (class. τοῖς μακρὰν [scil.
ὁδόν] ἀποικοῦσιν), 7:4, 12, 8: (20,) 23 (v.l.), 9:21 (ἐν all
witnesses but SA), 11:25 D, 14:25 (ἐν BCD), 17:13 D, 18:21 D, 19:22
(ἐν D), 21:23, 23:11 twice, 25:4, 26:20. The following are also
unclass.: Lk 1:44 ἐγένετο ἡ φωνὴ εἰς τὰ ὦτά μου and γενέσθαι
εἰς Ἰερ. A 20:16, 21:17, 25:15 (ἐν correctly 13:5). Homil Clem
12.10.2 εἰς Ῥώμην καταλείψας, 25.1 γενομένων ἡμῶν ‘since we had arrived (there)’.—Jn 17:23 ἵνα ὦσιν τετελειωμένοι εἰς (τὸ) ἕν
in which εἰς denotes rather the purpose, the result; 1 Jn 5:8 οἱ
τρεῖς εἰς τὸ ἕν εἰσιν belongs in §145(1). Kilpatrick (by
letter): A 4:5 συναχθῆναι ... εἰς (S, ἐν al.) Ἰ. ‘in Jerusalem’;
12:25 εἰς (BSð) Ἰερ. is to be taken with πληρώσαντες τὴν
διακονίαν (conjecture of Wescott-Hort; also K. Grobel by letter):
‘after they had delivered the relief offering in Jerusalem’ (Debrunner
prefers ‘after they had [brought the relief offering] to Jerusalem and
delivered it [there]; cf. R 15:31 ἡ διακονία ἡ εἰς Ἰ. For a
discussion of the literary problems involved here, s. Funk, JBL 75
(1956) 130–6; Dupont, NT 1 (1956) 275–303.—The corresponding class.
usage is ἔστη εἰς τὸ μέσον Jn 20:19, 26 (Xen., Cyr. 4.1.1); cf.
21:4 (v.l. ἐπί). With ὕπαγε νίψαι εἰς τὴν κολυμβήθραν 9:7 cf.
ἵν̓ αὐτὸ λούσῃ εἰς σκάφην Epict. 3.22.71, yet νίψαι appears
to be spurious (Lachmann; on A al., cf. 11). 1 P 3:20 εἰς ἣν
(κιβωτὸν) ὀλίγοι διεσώθησαν ‘within which a few were saved’; cf. 2
T 4:18 (LXX Gen 19:19).—Εἰς for ἐν is encountered also in the LXX
(s. supra), on Egyptian private documents, e.g. PTebt i 38.14 (113 bc)
εἰς ὃν ἐνοικεῖ ... οἶκον, BGU ii 385.5 (ii/iii ad) (ε)ἰς
Ἀλεξάνδρειαν ἐσσι (= ἐστι? or εἶσι??), 423.7 (ii ad)
κινδυνεύσαντος εἰς θάλασσαν, Epigr. Kaibel 134 (Imperial period)
εἰς τύνβον κεῖμαι.—Cf. Bauer s.v. 1 d β, 9.
- Interchange of εἰς and ἐν in a metaphorical sense.
(1) Temporal: Lk 1:20 πληρωθήσονται εἰς τὸν καιρὸν αὐτῶν, but
correct with ἐν Mt 21:41, 2 Th 2:6. Instrumental: A 7:53 ἐλάβετε
τὸν νόμον εἰς διαταγὰς ἀγγέλων = ἐν διαταγαῖς (cf. Mt 9:34 et
al.). Similarly the Hebrew לֵךְ לְשָׁלוֹם is rendered in Mk 5:34 and
Lk 7:50, 8:48 by ὕπαγε (πορεύου) εἰς εἰρήνην (so also the LXX, 1
Km 1:17 etc.), and in Ja 2:16 by ὑπάγετε ἐν εἰρήνῃ (as D does in
both references in Lk; Judg 18:6 B); the notion behind ἐν was
probably the loose associative dative (§198(2)).
(2) The variation is understandable where a Hebrew בְּ is translated,
to which the dative would correspond in classical Greek: thus with
πιστεύειν, ὀμνύναι, εὐδοκεῖν and especially with the rendering of
Hebrew בְּשֵׁם.
(3) ‘To do something to someone’ is expressed by ποιεῖν
(ἐργάζεσθαί) τι ἔν τινι, εἴς τινα, τινι (Attic τινα); s.
§157(1).
(4) With verbs of preaching, etc. both εἰς and ἐν are permissible
also in Attic if the message is directed to several (εἰπεῖν εἰς
τὸν δῆμον, ἐν τῷ δήμῳ); so also the NT κηρύσσειν εἰς Mk 13:10
(ἐν D), Lk 24:47, 1 Th 2:9 (ὑμῖν S✱), and ἐν 2 C 1:19, G 2:2,
εὐαγγελίζεσθαι εἰς 1 P 1:25, ἐν G 1:16.
Additional Notes
(1) Lk 13:9 κἄν ποιήσῃ καρπὸν εἰς τὸ μέλλον has class. pars.,
e.g. ἐς ὕστερον Hdt. 5. 74; also class. are A 13:42 εἰς τὸ
μεταξὺ σάββατον, 2 C 13:2 εἰς τὸ πάλιν (cf. class. εἰσαῦθις).
The temporal use of εἰς elsewhere accords fully with class. usage.
(2) For πιστεύειν (εἰς, ἐν, ἐπί, dat.) s. §187(6); also for
πεποιθέναι and ἐλπίζειν. Corresponding πίστις: ἡ ἐν Χρ. and ἡ
εἰς Χρ. besides obj. gen. Θαρρῶ ἐν ‘have confidence in’ 2 C 7:16,
but εἰς 10:1 = θρασύς εἰμι, ‘towards you’. Ὀμνύναι Mt 5:35 with
ἐν and εἰς side by side (class. acc., §149). Εὐδοκεῖν ‘be well
pleased’ often with ἐν; with εἰς Mt 12:18 OT (only ὃν S✱B, ἐν
ᾧ D), 2 P 1:17. Διστάζειν εἰς τὸν θεόν Herm Man 9.5 = οὐ
πιστεύειν.—Τῷ σῷ ὀνόματι (instr. dat.) is good Greek Mt 7:22
(12:21, s. §187(6)), Mk 9:38 AX al. (otherwise ἐν), Ja 5:10 AKL
(otherwise ἐν); cf. τῷ τῆς πόλεως ὀνόματι in an inscrip. from
Asia Minor of 37 ad (Deissmann, NBS 26 [BS 198]). Εἰς τὸ ὄνομά
τινος ‘on somebody’s account’ is generally vulgar Hell. (Deissmann,
LO4 97f. [LAE 121f.]; Rossberg 33, Mayser ii 2, 415), but in the NT,
if at all in this sense, only εἰς ὄνομα προφήτου etc. Mt 10:41f.,
εἰς τὸ ἐμὸν [p. 112] ὄνομα 18:20, (28:19,) with which the
Hebraism ἐν ὀνόματι κυρίου Mt 21:9 has fused (LXX always ἐν ὀν.
for בְּשֵׁם Psichari 202f.; Corssen, WkP 1919, 167f.). Once ἐπὶ τῷ
ὀνόματι μου Mt 18:5. With βαπτίζειν, εἰς τὸ ὄν. (A 8:16, 19:8)
may be used as well as ἐν τῷ ὀν. (10:48; 2:38 ἐν or ἐπί). On
the whole subject cf. also Witkowski, Bericht 74f.; Bietenhard, TW
s.v. ὄνομα.
(3) Ποιεῖν (τὸ) ἔλεος μετά (Hebr. עִם) τινος Lk 1:72, 10:37
(§227(3)) besides ποιεῖν ἐλεημοσύνας εἰς A 24:17. To express the
destination or use (‘for’) εἰς is good class. (δαπανᾶν εἰς), thus
e.g. Mk 8:19, 20 and λογεία, διακονία εἰς τοὺς ἁγίους 1 C 16:1,
2 C 8:4 etc.
(4) Mk 14:9 ὅπου ἐὰν κηρυχθῇ τὸ εὐαγγέλιον εἰς ὅλον τὸν
κόσμον, λαληθήσεται . . . could be a case of εἰς for ἐν.
- Other usages of εἰς. For εἰς in place of a predicate nominative s. §145, in place of a predicate accusative §157(5). But in G 3:14
ἵνα εἰς τὰ ἔθνη ἡ εὐλογία τοῦ Ἀβραὰμ γένηται the simple
case would be the dative (§189) or, more in accordance with classical
usage, the genitive (yet rather ‘in order that . . . might come among
the Gentiles’); cf. A 24:17, R 8:18, Herm Sim 8.3.2 and ἐγγίζειν
εἰς instead of τινί §193(2). In MGr εἰς is the periphrasis for the
missing dative. With γίνεσθαι εἰς cf. ἐλήλυθεν εἴς τινα ‘to come
to somebody (as a possession)’ in two papyri (Mayser ii 2, 406).
(1) Εἰς is also used for ἐπί and πρός: Jn 4:5 ἔρχεται εἰς
πόλιν ‘come to’, not ‘into’, Mt 12:41 μετενόησαν εἰς τὸ κήρυγμα
Ἰωνᾶ (cf. Hdt. 3.52 πρὸς τοῦτο τὸ κήρυγμα ‘at[; because of’, s.
Bauer s.v. 6a]). Causal εἰς: J. R. Mantey, JBL 70 (1951) 45–8,
309–12; R. Marcus, op. cit. 129f.; 71 (1952) 43f.
(2) With numbers εἰς is distributive = ‘-fold’: Mk 4:8 εἰς
τριάκοντα etc. (v.l. ἕν; W τὸ ἔν [likewise 20 three times] or
ἐν; s. §§248(3), 220(2)).
(3) Εἰς τέλος ‘fully’; εἰς κενόν ‘in vain’ Paul (classical διὰ
κενῆς).
Additional Notes
(1) Unclass. Lk 15:22 δότε δακτύλιον εἰς τὴν χεῖρα, class. for
which is περί; s. Plato, Rep. 2.359e. Likewise Lk 15:22 ὑποδήματα
εἰς τοὺς πόδας (dat. Hom., Od. 15.369); Ljungvik 32. Acc. to
Jeremias, ThZ 5 (1949) 230 an Aramaism. Εἰς of the recipients of a
message (Semitism): Mk 14:9 (for which Mt 26:13 has ἐν), Lk 24:47, 1
Th 2:9, A 17:15 D = Aram לְ. (cf. Black, Aramaic Approach 71 on Lk
4:43 D); Jeremias, ZNW 44 (1952/3) 100. Εἰς for ‘to, toward’: Jn
11:31, 38 ὑπάγει (ἔρχεται) εἰς (D 38 ἐπὶ) τὸ μνημεῖον, 20:3
(6 and 8 εἰς ‘into’); accordingly the v.l. of DHP Mk 3:7 ἀνεχώρησεν
εἰς (instead of πρὸς) τὴν θάλασσαν is acceptable, likewise in 2:13
Tdf. ἐξῆλθεν εἰς τὴν θ. with S✱ instead of παρά, 7:31 with SBDW
al; cf. εἰς τὸν ποταμόν ‘to the river’ and the like in the LXX,
Polyb. etc. (Kallenberg, RhM 66 [1911] 473ff.) and as early as Hdt.
4.200 ἀπίκατο εἰς τὴν πόλιν (which they afterwards besieged).
(2) MGr τἄφερε στὰ τριάντα acc. to Psichari 184; otherwise ἐπὶ
διηκόσια, τριηκόσια ‘up to . . .’ Hdt. 1. 193. Barale, Didaskaleion
2 (1913) 436ff. (PTebt i 39.33 [114 bc], 49.11 [113 bc] βλάβος εἰς
‘to derogate in value from . . .’).
(3) Εἰς τέλος 1 Th 2:16 ‘in full’, Lk 18:5 ἵνα μὴ εἰς τέλος
ἐρχομένη ὑπωπιάζῃ με ‘in order that she may not gradually (pres.
ὑπωπιάζῃ!) wear me out completely by her continued coming (pres.!)’
(cf. Klostermann, Hdb. in loc.; Oepke, TW ii 424.35f. ‘finally’), Jn
13:1 εἰς τέλος ἠγάπησεν αὐτούς ‘he gave them the perfect
love-token’ (Pernot, Études 207), LXX several times, Barn, Herm; εἰς
τέλος τουτέστι παντελῶς Diodorus Tars, on Ps 51:7 (MPG 33, 1589b).
Cf. Debrunner, Gnomon 4 (1928) 444, Bauer s.v. εἰς 3. Pap.
‘ultimately’ and ‘fully, completely’; Preisigke s.v. τέλος 4, Mayser
ii 2, 419, 570. Εἰς τέλος = יֶתֶר (עַל) ‘remainder,
exceedingly’.—Εἰς κενόν Diodor. and Hell. elsewhere, also LXX (with
Is 65:23 οὐ κοπιάσουσιν εἰς κ. cf. 1 Th 3:5 μήπως ... εἰς κ.
γένηται ὁ κόπος ἡμῶν; the Prophets have εἰς κενόν, Job and Ps
prefer διὰ κενῆς; side by side in Lev: διὰ κ. 26:16, εἰς κ.
26:20); Bauer s.v. κενός 2aβ. Following εἰς κ. the old equivalent
μάτην (also Mt 15:9, Mk 7:7, Herm Sim 5.4.2) is then expanded to εἰς
μάτην Herm Sim 5.4.2, 6.1.3, 9.4.8, 13.2 (LXX Ps 62:10, 126:1a, b, 2,
Lucian, Trag. 28.241).
(F. Blass, A. Debrunner, and Robert W. Funk, A Greek Grammar of the
New Testament and Other Early Christian Literature, Accordance
electronic ed. (Chicago: The University of Chicago Press, 1961),
110-112.)
When we realize that ⲉⲓⲥ is a very flexible word, we quickly realize that very often what drives the meaning (and thus translation) of the word in a bible translation, commentary or even a sermon is a front-shifting of theological conclusions.
Some conclude that baptism is an "outward sign of inner renewal." As such it is a pointer and just a sign with no power in and of itself.
Others hold onto the long-standing practice of taking ⲉⲓⲥ as a "telic" ⲉⲓⲥ. That is, the purpose of baptism is to convey/deliver/give the forgiveness of sins. This is the common and prevalent understanding of the phrase in the context of the Nicene Creed. The idea that one would be baptized just as a way of professing his/her faith was unknown to the ancients. For them there was one baptism. And in that baptism water was connected to all the power and promises of God's word for (i.e. delivering) the forgiveness of sins. The idea that a person would be baptized just as an 'outward expression of inner faith' is a fairly modern notion. Consider the words of the theologians in the early church:
The consistent conclusion of the Christian Church was that there was one baptism. And that baptism was water connected to the power and promises of God's word.
Might doesn't always make right. It might be tempting to recognize that since, throughout the long ages of the church, the common conclusion is that purpose (ⲉⲓⲥ) of baptism is the forgiveness of sins that conclusion must be right. But there are other doctrines that Church Fathers held to which are hard to support from the bible (e.g. exalting Mary as queen of the heavens, or co-redemptrix). However, it's much easier to make the case that the common and consistent conclusion of there being one baptism (water & word) for (i.e. the purpose of it) the forgiveness of sins is much easier to support than the elevation of Mary beyond what scripture can bear.
Due to the innate flexibility of the preposition, ⲉⲓⲥ, the word by itself cannot bear the weight of much of the meaning that is often placed on it. It's just too flexible on its own. As a result there is the necessity of drawing in the rest of the biblical context.
But there's the crux/rub/conundrum/problem. As soon as one draws in a larger context here in Hermeneutics SE, one is accused of having an agenda (usually by someone who clearly has an agenda of their own).
Note that this does not answer the question in the way you might have wanted. You wanted an "either/or" answer. I gave you this as an answer:
And just so that we're clear, I'm not saying in this context that bias/agenda is necessarily bad. I'm just saying that it's unavoidable.