Why does wounding a weak believer's conscience amount to sinning against Christ?
Wounding a weak believer's conscience is sin against Christ because that weak brother is one for whom Christ died. Using your knowledge in a way that causes him to stumble treats the Lord's sacrifice and care for him lightly. Christ understands the weak person's condition and holds the offender accountable for the harm done.
Les teaches that Paul addresses the believer who is grounded in the faith and has no compunctions, but must still consider the weak brother. Through the stronger believer's knowledge the weak brother can perish, for whom Christ died. When someone sins against the brethren and wounds their weak conscience, he sins against Christ. Les explains that the Lord understands where the weak person is and where the mature person is; He knows the mature has the wherewithal to make it a level playing field, so the mature must not cause the weak to stumble. Verse 13 says if meat makes a brother offend, the mature believer should eat no flesh while the world stands, lest he make his brother offend. Les connects this to Romans: if something does not hurt you but bothers a weak believer, for his sake do not do it. This lesson applies to many areas of life today.
But when ye sin so against the brethren, and wound their weak conscience, ye sin against Christ.