Have overcome them (νενικηκατε αυτους). Perfect active indicative of νικαω, calm confidence of final victory as in 1. John 2:13; John 16:33. The reference in αυτους (them) is to the false prophets in 1. John 4:1.

Because (οτ). The reason for the victory lies in God, who abides in them (1. John 3:20; 1. John 3:24; John 14:20; John 15:4). God is greater than Satan, "he that is in the world" (ο εν τω κοσμω), the prince of this world (John 12:31; John 14:30), the god of this age (2. Corinthians 4:4), powerful as he seems.