Step back and see the whole passage as one unit. Jesus says the wise man builds on the rock, but up to this point in the sermon he has never mentioned a rock. So what is the rock? We learn its identity later, in Matthew 16, when Peter confesses, you are the Christ, and Jesus answers, on this rock I will build my church. The rock is Christ himself. To build on the rock is to build on him. Everything this week has been pointing here.
And notice how the passage ends. The crowds were astonished at his teaching, for he was teaching them as one who had authority. The whole Sermon on the Mount forces one question: who is this who speaks with such authority? Matthew answered it in chapter one. This is the King, the son of David, the son of Abraham, the King who has finally arrived. The same King who speaks these words with all authority is the King who went to a cross and took the storm of God’s judgment in your place.
This is the gospel, and it turns the whole parable inside out. Building on the rock does not mean you finally perform well enough to come to him. It means the opposite. Remember the very first Beatitude that opened this whole series: blessed are the poor in spirit. To be poor in spirit is to come to God with empty hands, admitting you are a helpless sinner who cannot build anything that will stand against the final storm. Ortlund, in Gentle and Lowly, reminds us that we are naturally lawish, always trying to earn and keep score, but the heart of Christ is lavish, and our sin is like a few passing clouds that cannot put out the sun of his love.
So for everyone who belongs to Christ, hear the gospel declaration: the rock is not your record, it is his love; it is not your building, it is his cross. The King with all authority is the same King who took the flood of judgment for you, so that when the final storm comes, and it will, you will not fall, because you are founded on him. The Son of God loved you and gave himself for you. Build your whole life there.