Beloved, let us love one another: for love is of God; and every one that loveth is born of God, and knoweth God.
Resumption of the main theme (1 John 2:29). Love, the sum of righteousness, is the test of being born of God. Love flows from a sense of God's love. 1 John 4:9 resumes 1 John 3:16; 1 John 5:13 resumes 1 John 3:24. At the same time, 1 John 4:7-21 are connected with the preceding context, 1 John 4:2 setting forth Christ's incarnation, the great proof of God's love (1 John 4:10).
Beloved - appropriate to his subject, "love."
Love. All love is from God, its fountain; especially its great embodiment, God manifest in the flesh. The Father also is love (1 John 4:8). The Holy Spirit sheds love as its first-fruit abroad in the heart (Romans 5:5).
Knoweth God - spiritually, experimentally, habitually.