20 In this sentence the meaning is twofold. We must deal with it in both ways. For He who says, “I am not come to destroy the law, but to fulfil,” means it either in the way of adding what is wanting, or of doing what is in it. Let us then consider that first which I have put first: for he who adds what is wanting does not surely destroy what he finds, but rather confirms it by perfecting it; and accordingly He follows up with the statement, “Verily I say unto you, Till heaven and earth pass, one iota or one tittle shall in nowise pass from the law, till all be fulfilled.”
For, if even those things which are added for completion are fulfilled, much more are those things fulfilled which are sent in advance as a commencement. Then, as to what He says, “One iota or one tittle shall in nowise pass from the law,” nothing else can be understood but a strong expression of perfection, since it is pointed out by means of single letters, among which letters “iota” is smaller than the others, for it is made by a single stroke; while a “tittle” is but a particle of some sort at the top of even that.
And by these words He shows that in the law all the smallest particulars even are to be carried into effect. After that He subjoins: “Whosoever, therefore, shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven.” Hence it is the least commandments that are meant by “one iota” and “one tittle.” And therefore, “whosoever shall break and shall teach [men] so,”— i.e. in accordance with what he breaks, not in accordance with what he finds and reads—“shall be called the least in the kingdom of heaven;” and therefore, perhaps, he will not be in the kingdom of heaven at all, where only the great can be.
“But whosoever shall do and teach [men] so,” — i.e. who shall not break, and shall teach men so, in accordance with what he does not break—“shall be called great in the kingdom of heaven.” But in regard to him who shall be called great in the kingdom of heaven, it follows that he is also in the kingdom of heaven, into which the great are admitted: for to this what follows refers.
Source: Our Lord's Sermon on the Mount (New Advent)