A.T. Robertson Commentary 1 John 5:1

"Whosoever believeth that Jesus is the Christ is begotten of God: and whosoever loveth him that begat loveth him also that is begotten of him." — 1 John 5:1 (ASV)

That Jesus is the Christ (οτ Ιησους εστιν ο Χριστος). The Cerinthian antichrist denies the identity of Jesus and Christ (2:22). Hence John insists on this form of faith (πιστευων here in the full sense, stronger than in 3:23; 4:16, seen also in πιστις in verse 4, where English and Latin fall down in having to use another word for the verb) as he does in verse 5 and in accord with the purpose of John's Gospel (20:31). Nothing less will satisfy John, not merely intellectual conviction, but full surrender to Jesus Christ as Lord and Saviour. "The Divine Begetting is the antecedent, not the consequent of the believing" (Law). For "is begotten of God" (εκ του θεου γεγεννητα) see 2:29; 3:9; 4:7; 5:4,18. John appeals here to family relationship and family love.

Him that begat (τον γεννησαντα). First aorist active articular participle of γενναω, to beget, the Father (our heavenly Father).

Him also that is begotten of him (τον γεγεννημενον εξ αυτου). Perfect passive articular participle of γενναω, the brother or sister by the same father. So then we prove our love for the common Father by our conduct towards our brothers and sisters in Christ.