2 John 1:6

And this is love, that we should walk after his commandments. This is the commandment, even as ye heard from the beginning, that ye should walk in it.

In the writings of John, keeping God's commandments is the only proof, either of faith or of love. It is in such passages as these that the common authorship of all these epistles is so evident. See in 1 John 3:23 for further comment.