In these closing words of His sermon Jesus was stressing the importance of an adequate foundation, and He is asking the question, “What is your foundation? On what do you build?”
Now that is a most profound question. And it is a good one to come to at the end of the Sermon on the Mount. For, you see, it is quite possible for a man to have heard all these teachings of Jesus and to have said, “It is true. These are great sayings. They are the key to morality. I’ll just go out and try a bit harder.” But Jesus says that if you are thinking that way, you have missed the whole point of what He is saying. Jesus in effect says, “I am not asking you to go out and try harder. You will never be able to do it. To go out and try harder and to try to construct that kind of character in your strength is like trying to build a mansion upon sand. Actually, you will only achieve that kind of character when you build on Me.”
This is really the first and most important point of these verses. Jesus Christ is the foundation. He is the rock. I know, of course, that not all Scripture passages that use the word “rock” or “foundation” imply this, but certainly it is the true sense in this passage. It is true that in 1 Timothy 6:17-19, Paul speaks of works as a good foundation: “Charge them that are rich in this age… that they do good… laying up in store for themselves a good foundation against the time to come.” He also speaks of God’s eternal decree in election as our foundation: “Nevertheless, the foundation of God standeth sure, having this seal, The Lord knoweth them that are his” (2 Timothy 2:19). But these are exceptions, and for each of these texts there are many more that apply the same imagery to Jesus Himself or (in the Old Testament) to the Messiah.
Thus, Isaiah writes, “Therefore, thus saith the Lord God, Behold, I lay in Zion for a foundation a stone, a tested stone, a precious cornerstone, a sure foundation (Is. 28:16). Paul writes, “And ye are built upon the foundation of the apostles and prophets, Jesus Christ Himself being the chief cornerstone” (Eph. 2:20). Shortly after the resurrection Peter told the highest court of the Jews, the Sanhedrin, “This is the stone which was set at nought of you builders, which is become the head of the corner” (Acts 4:11). He wrote in his first letter, “Behold, I lay in Zion a chief cornerstone elect, precious; and he that believeth on him shall not be confounded. Unto you, therefore, who believe he is precious, but unto them who are disobedient, the stone which the builders disallowed, the same is made the headstone of the corner” (1 Peter 2:6-7).