Charles Spurgeon 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 whoever keeps his word, in him truly is the love of God perfected: by this we know that we are in him.

When every word of his is precious to us and when we strive to live according to his precepts, then we know that "we are in him."

This is even more than knowing that we know him, for it is the assurance that we are united to him by a living connection which can never be broken.

But whoso keepeth his word, in him verily is the love of God perfected: hereby know we that we are in him.

Note the gradation: we know him, we are in him, we know that we are in him.