Adam Clarke's Commentary on the Bible

Verse 6 (Romans 14:6)

He that regardeth the day - A beautiful apology for mistaken sincerity and injudicious reformation. Do not condemn the man for what is indifferent in itself: if he keep these festivals, his purpose is to honor God by the religious observance of them. On the other hand, he who finds that he cannot observe them in honor of God, not believing that God has enjoined them, he does not observe them at all. In like manner, he that eateth any creature of God, which is wholesome and proper food, gives thanks to God as the author of all good. And he who cannot eat of all indiscriminately, but is regulated by the precepts in the Mosaic law relative to clean and unclean meats, also gives God thanks. Both are sincere; both upright; both act according to their light; God accepts both; and they should bear with each other.

- Adam Clarke's Commentary on the Bible