1ST JOHN 5:20(B)

VINCENT'S WORD STUDIES by Marvin R. Vincent, 1886.
An understanding (διάνοιαν)
Only here in John's writings. The faculty of understanding. See on Luke 1:51. Westcott remarks that nouns which express intellectual powers are rare in the writings of John.
We may know (γινώσκομεν)
Apprehend progressively. Compare John 17:3.
Him that is true (τὸν ἀληθινόν)
Compare Revelation 3:7, Revelation 3:14; Revelation 6:10. On true, see on John 1:9. "God very strangely condescends indeed in making things plain to me, actually assuming for the time the form of a man, that I at my poor level may better see Him. This is my opportunity to know Him. This incarnation is God making Himself accessible to human thought - God opening to man the possibility of correspondence through Jesus Christ. And this correspondence and this environment are those I seek. He Himself assures me, 'This is life eternal, that they might know Thee, the only true God, and Jesus Christ whom Thou hast sent.' Do I not now discern the deeper meaning in Jesus Christ whom Thou hast sent? Do I not better understand with what vision and rapture the profoundest of the disciples exclaims, 'The Son of God is come, and hath given us an understanding, that we might know Him that is true?'" (Drummond, "Natural Law in the Spiritual World").
God the Father. Many, however, refer it to the Son.
Eternal life
See on 1 John 1:2.