[Heb 3:14 ASV] for we are become partakers of Christ, if we hold fast the beginning of our confidence firm unto the end:

What is he saying about "hold fast the beginning of our confidence firm unto the end"? To what does "our confidence" refer?

Notes:

I'm including BDAG's entry because this is not an obvious one:

ὑπόστασις, εως, ἡ (ὑφίστημι; Hippocr.+; Polyb. 4, 50, 10; 6, 55, 2; Diod S 16, 32, 3; 16, 33, 1; M. Ant. 10, 5; ins, pap, LXX; PsSol 15:5; 17:24; TestReub 2:7; TestZeb 2:4; Tat.; Ath. 21, 3; Iren. 5, 36, 1 [Harv. II 426, 1]; Hippol., Ref. 10, 17, 2; Did., Gen. 128, 11 in widely different meanings. See Dörrie 4 below.)
① the essential or basic structure/nature of an entity, substantial nature, essence, actual being, reality (underlying structure, oft. in contrast to what merely seems to be: Ps.-Aristot., De Mundo 4 p. 395a, 29f; Plut., Mor. 894b; Diog. L., Pyrrh. 9, 91; Artem. 3, 14; Ps 38:6; Wsd 16:21; TestReub 2:7; SJCh 78, 20; Philo, Aet. M. 88; 92; Jos., C. Ap. 1, 1; Tat. 6, 2; Ath. 21, 3; cp. the answer of a certain Secundus, who, when asked ‘Quid fides?’, answered: ‘ignotae rei mira certitudo’=a marvelous certainty about someth. otherwise unknown [FPhGr I 516]; s. also Lexicon Sabbaiticum: Lexica Graeca Minora ’65, 53)
ⓐ of the Son of God as χαρακτὴρ τῆς ὑποστάσεως αὐτοῦ a(n) exact representation of (God’s) real being (i.e. as one who is in charge of the universe) Hb 1:3. Sim. of polytheists’ deities, whose basic reality is someth. material like stone, metal etc. Dg 2:1.
ⓑ of things: among the meanings that can be authenticated for Hb 11:1 a strong claim can be made for realization (Diod S 1, 3, 2 of the realization of a plan; Cornutus 9 p. 9, 3 of the realization of humanity; Jos., C. Ap. 1, 1 that of the Jewish people, both by a divine act; Tat. 5, 1 of God τοῦ παντὸς ἡ ὑπόστασις): ἔστιν πίστις ἐλπιζομένων ὑπ.=in faith things hoped for become realized, or things hoped for take on (but s. 3 and 4 below) reality. Conversely, ‘without faith things hoped for would have no reality’. HKöster (s. bibliog. 4 below) argues for this sense also in 3:14, but s. 2. Cp. the rendering ‘substance’ (e.g. KJV, REB).
② a plan that one devises for action, plan, project, undertaking, endeavor (Diod. Sic 15, 70, 2; 16, 32, 3; 16, 82, 6; 17, 69, 7; Ezk 19:5) ἐν τῇ ὑποστάσει ταύτῃ in connection with this undertaking i.e. the collection for Jerusalem 2 Cor 9:4. The fact that meeting a financial obligation is the main theme (vss. 1–2) might well suggest association of ὑπ. with its use e.g. as a t.t. of expectation of rent due PTebt 61b, 194. To emphasize the importance of steadfast commitment to professed obligation (opp.   p 1041  καρδία πονηρὰ ἀπιστίας ἐν τῷ ἀποστῆναι), the author of Hb 3:14 uses ὑπ. in a way that invites an addressee to draw on the semantic component of obligation familiar in commercial usage of the term (s. PTebt above), an association that is invited by use of μέτοχος, a standard term for a business partner (PHib 109, 3; PCairZen 176, 102 [both III b.c.]), μέχρι τέλους (s.v. τέλος 2bβ), and βέβαιος (s. M-M s.v.). S. Köster 1b above for focus of ὑπ. on ‘reality’.—Satirically, ἐν ταύτῃ τῇ ὑποστάσει τῆς καυχήσεως in this boasting project of mine 2 Cor 11:17.
③ The interp. situation, condition (Cicero, Ad Attic. 2, 3, 3 ὑπόστασιν nostram=our situation), also specif. frame of mind (Dio Cass. 49, 9; Themist., Or. 13 p. 178b; Jos., Ant. 18, 24 of determination in desperate circumstances; sim. Polyb. 6, 55, 2) has been suggested for some of the passages cited in 1 and 2 above: 2 Cor 9:4 (explained in a v.l. via the epexegetical gen. καυχήσεως); 11:17; Hb 3:14 (s. Dörrie [bibliog. 4 below], p. 39: the frame of mind described in Hb 3:6). The sense ‘confidence’, ‘assurance’ (based on LXX [Ruth 1:12; Ps 38:8; Ezk 19:5], where it renders תִּקְוָה etc.) favored by Melanchthon and Luther (also Tyndale, NRSV, but not KJV) for Hb 11:1 has enjoyed much favor but must be eliminated, since examples of it cannot be found (s. Dörrie and Köster [4 below]). More prob. for Hb 4:11 is
④ guarantee of ownership/entitlement, title deed (Sb 9086 III, 1–11 [104 a.d.]; Spicq III 423 n. 14; cp. M-M s.v.) Hb 11:1 (cp. 2 above for commercial use of ὕπ.).—ASchlatter, Der Glaube im NT4 1927, 614ff; MMathis, The Pauline πίστις-ὑπόστασις acc. to Hb 11:1, diss. Cath. Univ. of Amer., Washington, D.C. 1920, also Biblica 3, 1922, 79–87; RWitt, Hypostasis: ‘Amicitiae Corolla’ (RHarris Festschr.) ’33, 319–43; MSchumpp, D. Glaubensbegriff des Hb: Divus Thomas 11, ’34, 397–410; FErdin, D. Wort Hypostasis, diss. Freiburg ’39; CArpe, Philologus 94, ’41, 65–78; HDörrie, Ὑπόστασις, Wort-u. Bedeutungsgeschichte: NAWG 1955, no. 3, ZNW 46, ’55, 196–202; HKöster, TW VIII 571–88 (Köster prefers plan, project [Vorhaben] for the passages in 2 Cor, and reality [Wirklichkeit] for all 3 occurrences in Hb, contrasting the reality of God with the transitory character of the visible world). S. also the lit. s.v. πίστις 2a.—DELG s.v. ἵστημι. M-M. EDNT. TW. Spicq. Sv.

Here it is with less formatting:

ὑπόστασις, εως, ἡ (ὑφίστημι; Hippocr.+; Polyb. 4, 50, 10; 6, 55, 2; Diod S 16, 32, 3; 16, 33, 1; M. Ant. 10, 5; ins, pap, LXX; PsSol 15:5; 17:24; TestReub 2:7; TestZeb 2:4; Tat.; Ath. 21, 3; Iren. 5, 36, 1 [Harv. II 426, 1]; Hippol., Ref. 10, 17, 2; Did., Gen. 128, 11 in widely different meanings. See Dörrie 4 below.) ① the essential or basic structure/nature of an entity, substantial nature, essence, actual being, reality (underlying structure, oft. in contrast to what merely seems to be: Ps.-Aristot., De Mundo 4 p. 395a, 29f; Plut., Mor. 894b; Diog. L., Pyrrh. 9, 91; Artem. 3, 14; Ps 38:6; Wsd 16:21; TestReub 2:7; SJCh 78, 20; Philo, Aet. M. 88; 92; Jos., C. Ap. 1, 1; Tat. 6, 2; Ath. 21, 3; cp. the answer of a certain Secundus, who, when asked ‘Quid fides?’, answered: ‘ignotae rei mira certitudo’=a marvelous certainty about someth. otherwise unknown [FPhGr I 516]; s. also Lexicon Sabbaiticum: Lexica Graeca Minora ’65, 53) ⓐ of the Son of God as χαρακτὴρ τῆς ὑποστάσεως αὐτοῦ a(n) exact representation of (God’s) real being (i.e. as one who is in charge of the universe) Hb 1:3. Sim. of polytheists’ deities, whose basic reality is someth. material like stone, metal etc. Dg 2:1. ⓑ of things: among the meanings that can be authenticated for Hb 11:1 a strong claim can be made for realization (Diod S 1, 3, 2 of the realization of a plan; Cornutus 9 p. 9, 3 of the realization of humanity; Jos., C. Ap. 1, 1 that of the Jewish people, both by a divine act; Tat. 5, 1 of God τοῦ παντὸς ἡ ὑπόστασις): ἔστιν πίστις ἐλπιζομένων ὑπ.=in faith things hoped for become realized, or things hoped for take on (but s. 3 and 4 below) reality. Conversely, ‘without faith things hoped for would have no reality’. HKöster (s. bibliog. 4 below) argues for this sense also in 3:14, but s. 2. Cp. the rendering ‘substance’ (e.g. KJV, REB). ② a plan that one devises for action, plan, project, undertaking, endeavor (Diod. Sic 15, 70, 2; 16, 32, 3; 16, 82, 6; 17, 69, 7; Ezk 19:5) ἐν τῇ ὑποστάσει ταύτῃ in connection with this undertaking i.e. the collection for Jerusalem 2 Cor 9:4. The fact that meeting a financial obligation is the main theme (vss. 1–2) might well suggest association of ὑπ. with its use e.g. as a t.t. of expectation of rent due PTebt 61b, 194. To emphasize the importance of steadfast commitment to professed obligation (opp. p 1041 καρδία πονηρὰ ἀπιστίας ἐν τῷ ἀποστῆναι), the author of Hb 3:14 uses ὑπ. in a way that invites an addressee to draw on the semantic component of obligation familiar in commercial usage of the term (s. PTebt above), an association that is invited by use of μέτοχος, a standard term for a business partner (PHib 109, 3; PCairZen 176, 102 [both III b.c.]), μέχρι τέλους (s.v. τέλος 2bβ), and βέβαιος (s. M-M s.v.). S. Köster 1b above for focus of ὑπ. on ‘reality’.—Satirically, ἐν ταύτῃ τῇ ὑποστάσει τῆς καυχήσεως in this boasting project of mine 2 Cor 11:17. ③ The interp. situation, condition (Cicero, Ad Attic. 2, 3, 3 ὑπόστασιν nostram=our situation), also specif. frame of mind (Dio Cass. 49, 9; Themist., Or. 13 p. 178b; Jos., Ant. 18, 24 of determination in desperate circumstances; sim. Polyb. 6, 55, 2) has been suggested for some of the passages cited in 1 and 2 above: 2 Cor 9:4 (explained in a v.l. via the epexegetical gen. καυχήσεως); 11:17; Hb 3:14 (s. Dörrie [bibliog. 4 below], p. 39: the frame of mind described in Hb 3:6). The sense ‘confidence’, ‘assurance’ (based on LXX [Ruth 1:12; Ps 38:8; Ezk 19:5], where it renders תִּקְוָה etc.) favored by Melanchthon and Luther (also Tyndale, NRSV, but not KJV) for Hb 11:1 has enjoyed much favor but must be eliminated, since examples of it cannot be found (s. Dörrie and Köster [4 below]). More prob. for Hb 4:11 is ④ guarantee of ownership/entitlement, title deed (Sb 9086 III, 1–11 [104 a.d.]; Spicq III 423 n. 14; cp. M-M s.v.) Hb 11:1 (cp. 2 above for commercial use of ὕπ.).—ASchlatter, Der Glaube im NT4 1927, 614ff; MMathis, The Pauline πίστις-ὑπόστασις acc. to Hb 11:1, diss. Cath. Univ. of Amer., Washington, D.C. 1920, also Biblica 3, 1922, 79–87; RWitt, Hypostasis: ‘Amicitiae Corolla’ (RHarris Festschr.) ’33, 319–43; MSchumpp, D. Glaubensbegriff des Hb: Divus Thomas 11, ’34, 397–410; FErdin, D. Wort Hypostasis, diss. Freiburg ’39; CArpe, Philologus 94, ’41, 65–78; HDörrie, Ὑπόστασις, Wort-u. Bedeutungsgeschichte: NAWG 1955, no. 3, ZNW 46, ’55, 196–202; HKöster, TW VIII 571–88 (Köster prefers plan, project [Vorhaben] for the passages in 2 Cor, and reality [Wirklichkeit] for all 3 occurrences in Hb, contrasting the reality of God with the transitory character of the visible world). S. also the lit. s.v. πίστις 2a.—DELG s.v. ἵστημι. M-M. EDNT. TW. Spicq. Sv.