John Chrysostom

By their "good," he means here either their faith, or the hope of rewards hereafter, or the perfectness of their religious state. For it is not only that you fail to profit your brother, he means, but the doctrine itself, and the grace of God, and His gift, you cause to be evil spoken of. Now when thou fightest, when thou quarrellest, when thou art vexatious, when thou makest schism in the Church, and reproachest thy brother, and art distant with him, those that are without will speak evil of you. …

Homily on Romans 26 ↗

Theophylact of Ohrid

That is, you have perfection in faith (for he called it "good"). Therefore do not abuse your perfection and do not give occasion for it to be blasphemed. For if you, being perfect, destroy the imperfect one, then you have given occasion to blaspheme the good that you possess. But you understand that our teaching is also good, and that it does not wish to be subjected to blasphemy from unbelievers when they see divisions and stumbling blocks concerning food.

Commentary on Romans ↗

Clement of Alexandria

For the supper is made for love, but the supper is not love; only a proof of mutual and reciprocal kindly feeling. "Let not, then, your good be evil spoken of; for the kingdom of God is not meat and drink," says the apostle, in order that the meal spoken of may not be conceived as ephemeral, "but righteousness, and peace, and joy in the Holy Ghost." He who eats of this meal, the best of all, shall possess the kingdom of God, fixing his regards here on the holy assembly of love, the heavenly Church.

The Instructor Book 2 ↗

Thomas Aquinas

Then when he says, let not then, he presents the third reason, which is taken on the part of the gifts of spiritual grace. First, he shows what impropriety follows against such gifts from the fact that we place a stumbling block; second, he clarifies what he had said, at for the kingdom of God; third, he draws a conclusion, at therefore, let us follow after the things that are of peace. …

Commentary on Romans ↗