1 John 1:8

This second claim (cf. 1 John 1:6) is more serious, and its results are worse: we do not just lie, but we deceive ourselves.

If a Christian claims to be enjoying fellowship with God, he may think he is temporarily or permanently entirely sinless. Yet our sinfulness exceeds our consciousness of guilt. We have only a very limited appreciation of the extent to which we sin. We commit sins of thought as well as deed, sins of omission as well as commission, and sins that spring from our nature as well as from our actions.

Some have interpreted the phrase "no sin" to mean no sin nature or no sin principle. [Note: E.g., Smalley, p. 29.] However this seems out of harmony with John’s other uses of "to have sin" (cf. John 15:22; John 15:24; John 19:11). Rather, it probably means to have no guilt for sin. [Note: Robert Law, The Tests of Life: A Study of the First Epistle of St. John, p. 130; Robertson, 6:208.]

God’s truth, as Scripture reveals it, does not have a full hold on us, is not controlling our thinking, if we make this claim. "In us" suggests not that we have the facts in our mental grasp but that they have control over us. They are in us as a thread is in a piece of cloth rather than as a coin is in a pocket. The same contrast exists between intellectual assent and saving faith.