Everyone who believes with an upright and pure heart, and confesses such faith with his mouth and whole life (cf. Rom 10:9), that Jesus of Nazareth, the son of Mary, is the Christ, the promised Savior and Redeemer of the world, and the true Son of God (1 Jn 4:15; 1 Jn 2:22; 1 Jn 4:2-3), everyone who believes thus has been born of God (Jn 1:13; 1 Jn 2:29; 1 Jn 3:9).
That is, he has been regenerated by grace, so that he has become a true child of God (1 Jn 3:1; 1 Jn 2:21 and note). Thus in birth from God lie the foundation and source of faith, and only the children of God can be believers in the Son of God.
Faith in Christ and sonship to God are correlative concepts and identical in their correlation. One cannot believe in Christ as the true Son of God and Savior of the world, and confess Him, without being regenerated by God through the Holy Spirit and without being a child of God.
Nor can one truly be born, or regenerated, of God and not believe and confess Jesus as the Christ, the true Son of God.
“If we have received from the Teacher the commandment to love one another, then without doubt, if we believe that our Teacher Jesus is the Christ, and that He is Christ in the sense that He is God and at the same time man, we must also keep His commandments, as Teacher and as God.
And believing that He is God, we receive the name of His children, as is also said in the Gospel (Jn 1:12)” (Theophylact).
Everyone who has such living and firm faith in Christ Jesus necessarily includes in his consciousness this proposition: he who loves the One Who begot, that is, God, the common heavenly Father of all, also loves with active and strong love the one begotten of Him.
This means his fellow brother according to spiritual birth, or regeneration. Thus, in his consciousness and by the very essence of the matter, love for God and love for neighbor are inseparable in regeneration.
He who loves God cannot fail to love his neighbor. He who loves his neighbor cannot fail to love God. The inner bond uniting them is sonship to God.
“If we have been born of Him, then without doubt we must render what is due to the Parent, that is, love the One Who begot. And if all of us who have believed have been born of Him, then we must love one another both as brothers and as those born of Him” (Theophylact).
This lofty truth the apostle persistently affirms and explains in different combinations throughout his epistle, and with different shades of thought, as here in the expression under consideration.