Not by this or from this, as an inference (see on 1Jo 4:6), but in the very exercise of the sentiment toward God, we perceive.
When (ὅταν)
More strictly, whenever. Our perception of the existence of love to our brethren is developed on every occasion when we exercise love and obedience toward God.
Keep (τηρῶμεν)
Read ποιῶμεν do. So Rev. See on Joh 3:21; see on 1Jo 3:4. The exact phrase ποιεῖν τὰς ἐντολὰς to do the commandments, occurs only here. See on Rev 22:14.