87 Hence we must take special notice how terribly the conclusion of the whole sermon is introduced: “Therefore, whosoever hears these sayings of Mine, and does them, is like unto a wise man, which built his house upon the rock.” For no one confirms what he hears or understands, unless by doing. And if Christ is the rock, as many Scripture testimonies proclaim that man builds in Christ who does what he hears from Him. “The rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell not: for it was founded upon a rock.”
Such an one, therefore, is not afraid of any gloomy superstitions (for what else is understood by rain, when it is put in the sense of anything bad?), or of turnouts of men, which I think are compared to winds; or of the river of this life, as it were flowing over the earth in carnal lusts. For it is the man who is seduced by the prosperity that is broken down by the adversities arising from these three things; none of which is feared by him who has his house founded upon a rock, i.e. who not only hears, but also does, the Lord's commands.
And the man who hears and does them not is in dangerous proximity to all these, for he has no stable foundation; but by hearing and not doing, he builds a ruin. For He goes on to say: “And every one that hears these sayings of Mine, and does them not, shall be like a foolish man, which built his house upon the sand: and the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell: and great was the fall of it. And it came to pass, when Jesus had ended these sayings, the people were astonished at His doctrine: for He taught them as one having authority, and not as their scribes.” This is what I said before was meant by the prophet in the Psalms, when he says: “I will act confidently in regard of him.
The words of the Lord are pure words: as silver tried and proved in a furnace of earth, purified seven times.” And from this number, I am admonished to trace back those precepts also to the seven sentences which He has placed in the beginning of this sermon, when He was speaking of those who are blessed; and to those seven operations of the Holy Spirit, which the prophet Isaiah mentions; but whether the order before us, or some other, is to be considered in these, the things we have heard from the Lord are to be done, if we wish to build upon a rock.
Source: Our Lord's Sermon on the Mount (New Advent)