Clearly from experiential reality, Christians are not omniscient, so that cannot be the point of 1 John 2:20.
John's writings do reflect regular uses of the neuter plural form of the adjective πᾶς used as a direct object of a verb for seeing/knowing (such as οἶδα or γινώσκω). And BDAG makes clear that the accusative plural πάντα has the idea of "all things" or "everything."
But it is a mistake to assume that "all things" or "everything" is necessarily a universal statement, truly meaning ALL THINGS (but it may mean that). Context still determines the limits of the "all" as it does in any situation. So as an example, John 18:4 was not given (for good reason) by the OP (English is NKJV):
4 Jesus therefore, knowing all things that would come upon Him, went forward and said to them, “Whom are you seeking?”
Ἰησοῦς οὖν, εἰδὼς πάντα τὰ ἐρχόμενα ἐπʼ αὐτόν, ἐξελθὼν εἶπεν αὐτοῖς, Τίνα ζητεῖτε;
In that context, the εἰδὼς πάντα has the "all" functioning as an adjective, not a substantive, limiting the τὰ ἐρχόμενα ἐπʼ αὐτόν. So it is everything "that would come upon Him" that is known.
But when πάντα is substantively used by itself, there may still be a contextual limit to what the "all things" is referring to. That is what needs to be determined. "All" always means "all," but it only refers to the whole of whatever is being referred to.
So does context reveal anything more that might limit any of these "all things"?
While the verse may be an acknowledgement of universal knowledge the disciples believe Christ to have, it may also be that they refer to "all things" Christ has just spoken to them about regarding His leaving them. That was the primary topic from verses 16-30, their question on their mind about what He means about seeing and not seeing Him and only for a "little while" (v.16-18). Christ's explanation and expansion is in v.19-27, the very last part of which indicates He speaks of going out of the world to the Father (v.28), which is plain enough for the disciples to understand (v.29). So "all things" related to His coming and going from the world may be the implicit referent that limits the "all" in this context.
The context here could implicitly have "all" limited to the points Jesus just expressed, namely that if Peter does love Him, he will do the feeding (v.15) and tending (v.16) that were just requested of Him based on Peter's previous two testimonials of loving Jesus. So "you know all" may be Peter stating he is aware Christ knows Peter will do what He asks.
However, this passage does lend a bit more credence to a universal all being acknowledged by Peter in his statement. There are two contextual items especially that lean that way:
- The form of Peter's reply in v.17: "Lord, You know all things; You know that I love You." This seems to be an argument of greater to lesser. That is, Peter is saying, "Lord, You know all things [i.e., EVERYTHING THERE IS TO KNOW]" (the greater), so therefore (the lesser) "You know that I love you."
- After Jesus expresses some future knowledge of Peter (v.18), Peter questions Jesus about another man's future (v.21, John's), showing that Peter knows Jesus knows their futures.
So a universal "know all things" has more weight in this passage. Let's skip 1 John 2:20 for a moment, and move to...
1 John 3:20
The context here indicates that the "knows all things" refers to the fact that what we know in our heart about our actions and motivations, God knows (v.18-20), for "God is greater than our heart." So the "all things" is "all things" of peoples hearts. While this may not be a reference to God's full omniscience, that God can know things individuals keep to themselves in their innermost being is certainly a subset of full omniscience.
1 John 2:20
The context of this verse is a lead in from John that there is an Antichrist to come (v.18a), but even now there are antichrists who have come (v.18b). These antichrists "went out from us" because they were "not of us" (v.19).
Then he says to those who have remained, the fact that they have remained shows that they have "an anointing from the Holy One" (v.20a) such that they "have continued with us" (v.19b), and with this anointing, John declares they "know all things." But John can mean a few possible things:
- Because of the anointing they have gained a knowledge of all things
- Because of a knowing all things they have gained the anointing
- Circumstantially, the anointing and the knowledge come at the same time
- The knowledge is simply a tacit admission from John based on their anointing
That is, the "and" that links the knowledge to the anointing simply shows there is some relationship between the two, but as of this point, John has not clarified what that relation may be. So let me describe further what I mean by the four possibilities, in reverse order:
As to #4, John may be saying "all things" in relation to what verses 18-19 refer to, the coming and outcome of the antichrists. John may simply be acknowledging that he knows they are annointed and he knows they know all these things regarding antichrists coming and leaving among them. This fits the context.
As to #3, that would mean that at the same time they were annointed they were given knowledge of all things (in some universal sense). As I opened here, experience shows that is not the case.
As to #2, this has merit of "know all" in a proleptic sense, for the following verse (v.21) may well be expanding on the "all" John is referring to. Those that have been anointed were anointed because they "know the truth," all the truth that they need to know to have gained the anointing and prove they are not antichrists, that they will not deny the Son (v.22-23). So contextually, this possibility also fits.
This "all things" being a reference to truth tends to be the normal interpretation of those that see πάντα (rather than πάντες) in this verse. A sampling of commentaries show this (bolding added):
H. D. M. Spence-Jones, ed., 1 John, The Pulpit Commentary (London; New York: Funk & Wagnalls Company, 1909), 27 (NOTE: as the quote shows, he is not totally decided):
It is hard to decide between three readings: (1) καὶ οἴδατε πάντα, “and ye know all things” necessary to salvation, i.e. “the truth” (ver. 21; John 16:13); (2) καί οἴδατε πάντες, “and ye all know” that ye have this anointing; (3) οἴδατε πάντες, “ye all know—I did not write to you because ye know not the truth.” There is evidence of a fourth variation, πάντας, “ye know all” the antichrists. If (1) be right, it does not mean that the Christian is omniscient, but that he has the basis of all knowledge; he can see things in their right proportions.
D. Edmond Hiebert, “An Expositional Study of 1 John Part 4 (of 10 Parts): An Exposition of 1 John 2:18–28,” Bibliotheca Sacra 146 (1989): 84 (NOTE: Hiebert does not support the TR reading):
The Textus Receptus reading certainly cannot mean that the readers “know all things”; but those supporting the reading reply that “all things” is limited by “the truth.”
Daniel L. Akin, 1, 2, 3 John, vol. 38, The New American Commentary (Nashville: Broadman & Holman Publishers, 2001), 119 (NOTE: He support TR reading):
The verse concludes with the statement that all believers know the truth (“the truth” is not in the Gk. text but is supplied from v. 21; see also the NLT). John seeks to encourage these believers by explaining that they are the ones who truly know God and that it is this knowledge of God that enables them to understand the false nature of what the heretics are teaching.
Gary W. Derickson, First, Second, and Third John, edited by H. Wayne House, W. Hall Harris III, and Andrew W. Pitts, Evangelical Exegetical Commentary (Bellingham, WA: Lexham Press, 2012), 236 (NOTE: They support the TR reading):
“All things” does not mean “everything.” This is not a promise of omniscience, an incommunicable divine attribute. Rather, it describes what one needs to know to resist the false teachers (2:21). The church of John’s day was already faced with false doctrines involving the person and work of Christ. Gnosticism was in its early stages of development and would blossom to a major controversy in the following century. Gnostic Christians claimed to receive secret knowledge of the truth. This knowledge was necessary for salvation, and only the élite could know these secret truths and thus have salvation. John opposed this concept by affirming that believers know the truth, everything they need to know. Furthermore, what John is writing is not something new or secret but something known by all.
As to #1, that would mean that because they were annointed they were given knowledge of all. This could not be in the universal sense (else a similar problem to #3), but also has a challenge as being a cause to know the truth (i.e., opposite of #2) for the whole premise of the argument is that knowing the truth of the Son's relation to the Father = accepting that truth = why they are anointed, as v.24 also relates implicitly (and cf. John 8:31-32).
Therefore let that [i.e. the truth] abide in you which you heard from the beginning. If what you heard from the beginning abides in you, you [i.e., by the anointing] also will abide in the Son and in the Father.
While it cannot be fully dismissed from possibility that #4 is the intention in 1 John 2:20, that John is simply acknowledging he knows they know these things about the antichrists, context to me favors #2: that "knows all things" is limited in the next verses to be referring to all the necessary truths of the relation of the Son to the Father that bring one to abiding in that truth and thus in the Son and the Father. Additionally, commentaries indicate that those who take v.20 to read πάντα follow similar logic to arrive at the same conclusion.
But it is clear that John does not use "knoweth all things" in any fixed sense. It is a favorite phrase of his, but the context limits what he means by the "all things" in any particular passage.
1 This answer works under the assumption that the Majority text reading of an accusative case πάντα is in 1 John 2:20. That seems to be the key crux of the question here, and my textual philosophy follows the Majority text reading as reflective of the original text. According to the NA28 apparatus the witnesses for that reading are:
A C 5. 33. 81. 307. 436. 442. 642. 1175. 1243. 1448. 1611. 1735. 1739. 1881. 2344. 2492 Byz latt sy; CyrJ Did
The minority reading of a nominative case πάντες that NA28 itself follows is witnessed by:
א B P Ψ 1852 sy sa; Hes
2 If I say: "The apples are in the barrel. I know all are good." The hearer knows my substantive "all" is not inclusive of everything that exists by the context of what was stated around it, specifically my mention of apples in the barrel prior to the "all" statement. So the "all" is limited to the whole of the apples implicitly by context alone, rather than by a direct, explicit adjectival statement such as "I know all the apples are good."