Albert Barnes Commentary 1 Corinthians 8:12

"And thus, sinning against the brethren, and wounding their conscience when it is weak, ye sin against Christ." — 1 Corinthians 8:12 (ASV)

But when you sin so against the brothers. This is designed further to show the evil of causing others to sin, and thus the evil which might arise from partaking of the meat offered to idols. The word sin here is to be taken in the sense of injuring, offending, or leading into sin. You violate the law which requires you to love your brothers and to seek their welfare, and thus you sin against them. Sin is properly against God; but there may be a course of injury pursued against people, or doing them injustice or wrong, and this is sin against them. Christians are bound to do right towards all.

And wound their weak conscience. The word "wound" here (tuptontes, meaning smiting, beating) is taken in the sense of injure. Their consciences are ill-informed. They do not have the knowledge which you have. And by your conduct they are led further into error, and believe that the idol is something and is to be honored. They are thus led into sin, and their conscience is more and more perverted, and oppressed more and more with a sense of guilt.

You sin against Christ. Because,

1. Christ has commanded you to love them, and seek their good, and not to lead them into sin; and,
2. because they are so intimately united to Christ (see Barnes on John 15:1 and following) that to offend them is to offend him; to injure the members is to injure the Head; to destroy their souls is to pain his heart and to injure his cause (see Barnes on Matthew 10:40).

Compare Luke 10:16.