If we say that we have no sin, we deceive ourselves, and the truth is not in us.
If we say ... is an expression of remarkable interest, because the apostle here identified himself with the false teachers, not through any agreement with them, but out of a delicate regard for his readers.  This identification of an apostle with those addressed is prevalent in the New Testament.   Hebrews 2:3 is a remarkable example of the same thing; and yet that instance of it has been perverted to mean that no first generation Christian could have written that epistle!