Commentaries:
Robertson's Word Pictures (NT)
Is come (hkei). Present active indicative, but the root has a perfect sense, "has come." See echlqon kai hkw in John 8:42.
An understanding (dianoian). Here alone in John's writings, but in Paul (Ephesians 4:18) and Peter (I Peter 1:13). John does not use gnwsiv (knowledge) and nouv (mind) only in Revelation 13:18; Revelation 17:9.
That we know (ina ginwskomen). Result clause with ina and the present active indicative, as is common with ina and the future indicative (John 7:3). It is possible that here o was pronounced w as a subjunctive, but many old MSS. have ina ginwskousin (plainly indicative) in John 17:3, and in many other places in the N.T. the present indicative with ina occurs as a variant reading as in John 5:20.
Him that is true (ton alhqinon). That is, God. Cf. I John 1:8.
In him that is true (en tw alhqinw). In God in contrast with the world "in the evil one" (verse I John 5:19). See John 17:3.
Even in his Son Jesus Christ (en tw uiw autou Ihsou Xristw). The autou refers clearly to en tw alhqinw (God). Hence this clause is not in apposition with the preceding, but an explanation as to how we are "in the True One" by being "in his Son Jesus Christ."
This (outov). Grammatically outov may refer to Jesus Christ or to "the True One." It is a bit tautological to refer it to God, but that is probably correct, God in Christ, at any rate. God is eternal life (John 5:26) and he gives it to us through Christ.
Other Robertson's Word Pictures (NT) entries containing 1 John 5:20:
1 Corinthians 4:6
Galatians 4:17
1 John 5:18
2 John 1:7
Revelation 12:6
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