Albert Barnes Commentary 1 John 2:5

"but whoso keepeth his word, in him verily hath the love of God been perfected. Hereby we know that we are in him:" — 1 John 2:5 (ASV)

But whoso keepeth his word. That is, what He has spoken or commanded. The term word here includes all that He has made known to us as His will regarding our conduct.

In him verily is the love of God perfected. One who professes to have the love of God in his heart finds that love receives its completion or filling up through obedience to the will of God. That obedience is the proper expression, or the exponent, of the love that exists in the heart. Love for the Savior would be defective without it, for love is never complete without obedience.

If this is the true interpretation, then the passage does not make any affirmation about sinless perfection. Instead, it only affirms that if true love exists in the heart, it will be carried out in life. Alternatively, it affirms that love and obedience are parts of the same thing: one will be manifested by the other, and where obedience exists, it is the completion or perfecting of love.

Furthermore, the apostle does not say that either the love or the obedience would be, in themselves, absolutely perfect; but he says that one cannot fully develop without the other.

Hereby know we that we are in him. That is, by having, in fact, such love as ensures obedience. To be in Him is to be united to Him, to be His friends (John 6:56; Romans 13:14).