Adam Clarke on 2 John 1:7

For many deceivers, etc. - Of these he had spoken before, see 1Jn 4:1, etc. And these appear to have been Gnostics, for they denied that Jesus was come in the flesh. And this doctrine, so essential to salvation, none could deny but a deceiver and an antichrist. Instead of εισηλθον are entered in, many excellent MSS. and versions have εξηλθον, are gone out. The sense is nearly the same.