Whosoever believeth that Jesus is the Christ is born of God: and every one that loveth him that begat loveth him also that is begotten of him.
Why our "brother" (1 John 4:21) is entitled to such love, namely, because he is 'begotten of God:' so, if we want to show love to God, we must show it to God's representative.
Whosoever, х Pas (G3956) ho (G3588)] - 'Everyone that.' He could not be our "Jesus" (God-Saviour) unless He were "the Christ;" for He could not reveal the way of salvation except He were a prophet: He could not work it out except He were a priest: He could not confer it upon us except He were a king. He could not be prophet, priest, and king, except He were the Christ (Pearson 'On the Creed').
Born - "begotten," as in end of the verse. Christ is the "only-begotten Son" by generation: we become begotten sons of God by regeneration and adoption. Parallel to John 3:3; John 3:5.
Everyone that loveth him that begat - sincerely; not in mere profession (1 John 4:20).
Loveth him also that is begotten - `his brethren' (1 John 4:21).