The Enchiridion by St. Augustine
Chapter 79.--Sins Which Appear Very Trifling, are Sometimes in Reality Very Serious.
Again, there are some sins which would be considered very trifling, if the Scriptures did not show that they are really very serious. For who would suppose that the man who says to his brother, |Thou fool,| is in danger of hell-fire, did not He who is the Truth say so? To the wound, however, He immediately applies the cure, giving a rule for reconciliation with one's offended brother: |Therefore, if thou bring thy gift to the altar, and there rememberest that thy brother hath ought against thee; leave there thy gift before the altar, and go thy way: first be reconciled to thy brother, and then come and offer thy gift.| Again, who would suppose that it was so great a sin to observe days, and months, and times, and years, as those do who are anxious or unwilling to begin anything on certain days, or in certain months or years, because the vain doctrines of men lead them to think such times lucky or unlucky, had we not the means of estimating the greatness of the evil from the fear expressed by the apostle, who says to such men, |I am afraid of you, lest I have bestowed upon you labor in vain|?