Jamieson, Fausset and Brown on 1 John 4:7

7. Resumption of the main theme (1Jo 2:29). Love, the sum of righteousness, is the test of our being born of God. Love flows from a sense of God's love to us: compare 1Jo 4:9 with 1Jo 3:16, which 1Jo 4:9 resumes; and 1Jo 4:13 with 1Jo 3:24, which similarly 1Jo 4:13 resumes. At the same time, 1Jo 4:7-21 is connected with the immediately preceding context, 1Jo 4:2 setting forth Christ's incarnation, the great proof of God's love (1Jo 4:10).

Beloved--an address appropriate to his subject, "love."

love--All love is from God as its fountain: especially that embodiment of love, God manifest in the flesh. The Father also is love (1Jo 4:8). The Holy Ghost sheds love as its first fruit abroad in the heart.

knoweth God--spiritually, experimentally, and habitually.