Jamieson, Fausset and Brown on 1 John 4:3

3. confesseth not that Jesus Christ is come in the flesh--Irenæus [3.8], Lucifer, Origen, on Mt 25:14, and Vulgate read, "Every spirit which destroys (sets aside, or does away with) Jesus (Christ)." Cyprian and Polycarp support English Version text. The oldest extant manuscripts, which are, however, centuries after Polycarp, read, "Every spirit that confesseth not (that is, refuses to confess) Jesus" (in His person, and all His offices and divinity), omitting "is come in the flesh."

ye have heard--from your Christian teachers.

already is it in the world--in the person of the false prophets (1Jo 4:1).