1 John 5:10

5:10 (The one who believes in the Son of God has the testimony in himself; the one who does not believe God has made him a liar, because he has not believed in the testimony that God has testified concerning his Son.) ¹ 

1 John 5:20

5:20 And we know that the Son of God has come and has given us insight to know ²  him who is true, and we are in him who is true, in his Son Jesus Christ. This one ³  is the true God and eternal life.

[5:10]  ¹ sn This verse is a parenthesis in John’s argument.

[5:20]  ² tn The ἵνα (Jina) introduces a purpose clause which gives the purpose of the preceding affirmation: “we know that the Son of God has come and has given us insight (so that we may) know him who is true.”

[5:20]  ³ sn The pronoun This one (οὗτος, Joutos) refers to a person, but it is far from clear whether it should be understood as a reference (1) to God the Father or (2) to Jesus Christ. R. E. Brown (Epistles of John [AB], 625) comments, “I John, which began with an example of stunning grammatical obscurity in the prologue, continues to the end to offer us examples of unclear grammar.” The nearest previous antecedent is Jesus Christ, immediately preceding, but on some occasions when this has been true the pronoun still refers to God (see 1 John 2:3). The first predicate which follows This one in 5:20, the true God, is a description of God the Father used by Jesus in John 17:3, and was used in the preceding clause of the present verse to refer to God the Father (him who is true). Yet the second predicate of This one in 5:20, eternal life, appears to refer to Jesus, because although the Father possesses “life” (John 5:26, 6:57) just as Jesus does (John 1:4, 6:57, 1 John 5:11), “life” is never predicated of the Father elsewhere, while it is predicated of Jesus in John 11:25 and 14:6 (a self-predication by Jesus). If This one in 5:20 is understood as referring to Jesus, it forms an inclusion with the prologue, which introduced the reader to “the eternal life which was with the Father and was manifested to us.” Thus it appears best to understand the pronoun This one in 5:20 as a reference to Jesus Christ. The christological affirmation which results is striking, but certainly not beyond the capabilities of the author (see John 1:1 and 20:28): This One [Jesus Christ] is the true God and eternal life.