That knowledge. Although most of the Corinthian believers might, indeed, understand an idol to be nothing at all, and that there is only one God, it was difficult for some to throw off immediately all their former superstitions and moods.
With conscience. Textual evidence is divided (cf. p. 10) between this and the reading “according to habitual use,” or “from force of habit.” With the latter reading the passage may be translated, “But some, through being hitherto accustomed to idols” (RSV). There were some among the church members who could not regard food that had been sacrificed to idols as ordinary food, even though they no longer believed in the existence of idols. As a result of lifelong custom they could not completely dissociate themselves from the past. Partaking of such food placed them vividly in their former setting, a situation that was more than they could bear.
Being weak. The conscience was not sufficiently strong to enable such persons to overcome all their former prejudices and superstitious beliefs.
Defiled. It is defiled because it is violated. Whatsoever is done without a clear conscience is sin (see on Rom. 14:23).