Commentaries:
John calls the various individuals who were teaching heresy "antichrists." At one time, these people had fellowshipped with true believers, but then had left the church and were now trying to draw others away to follow their heretical teachings. John points out that they were never really converted, or they would have stayed with the body of true believers.
Earl L. Henn
For the Perfecting of the Saints
John informs us that the antichrists were right in the church fellowshipping with the truly converted! No doubt, they performed the same function in John's areas of responsibilities as they did in Paul's. They created a measure of havoc in the church through heretical teaching and then left the fellowship, proving they were not really part of the church. They were tares.
John W. Ritenbaugh
God's Sovereignty and the Church's Condition (Part Two)
Other Forerunner Commentary entries containing 1 John 2:19:
Matthew 13:38
Galatians 1:6