The natural human reaction would be to admonish and discipline the weak, sinning brother. Paul, however, emphasizes the sin of the other brother, who has acted from knowledge
rather than from love. By putting a stumbling block
in his brother’s way (1 Corinthians 8:9), he has sinned not only against his brother, but ultimately against Christ himself.
12 Thus, sinning against your brothers and wounding their conscience when it is weak, you sin against Christ.