1 John 3:19-20

"By this" refers to what John said in 1 John 3:17-18. Tangible demonstrations of love for the brethren show the believer’s true character, his righteousness. They should be a comfort to us when we feel guilty that we have not met many needs, a condition that prevails no matter how generous we may be. We can overcome feelings of false guilt by remembering that God knows our real motives. He does not judge on the basis of appearance, as we often judge ourselves.

"This phrase ["before Him," 1 John 3:19] could refer to standing in the presence of God on the day of judgment (1 John 4:17), an occasion which might well fill the heart of a man with foreboding. But the context here is one of prayer: dare we approach God with our requests if we feel guilty before him? On the whole, it seems more likely that this is what is in John’s mind (cf. 1 Thessalonians 1:3; 1 Thessalonians 3:9[-10]). We then have a smooth transition to 1 John 3:21. [Note: Marshall, p. 199.]