1 John 2:5

But whoever keeps his word, truly in him the charity of God is perfected. And by this we know that we are in him

Other versions

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

but whoever keeps his word, in him truly the love of God is perfected. By this we may be sure that we are in him:

But if anyone obeys his word, God's love is truly made complete in him. This is how we know we are in him: