Commentaries:
John makes it clear in verses 20-21 that his audience is composed of deeply converted people who have God's Holy Spirit and are rooted and grounded in the truth. The "anointing" is the gift of the Holy Spirit, as we can see from II Corinthians 1:21-22: "Now He who establishes us with you in Christ and has anointed us is God, who also has sealed us and given us the Spirit in our hearts as a deposit." John's phrase, "you know all things," simply refers to the fact that these people knew and understood the basic doctrines of the church.
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 2:20:
Galatians 1:6