SOM Part 24 can be found here.
Jesus has just finished saying that not everyone who says calls Him Lord will enter the kingdom of heaven, (Matthew 7:21) He is now going to illustrate this with His conclusion. He talks about 2 houses, It would seem that to look at the houses, there would be little difference. The difference is in what is not readily seen from the outside, that being the foundation. What are the houses dependent upon.
The house on the sand appears to be dependent solely on its outward appearance. Perhaps much like the people who call Jesus Lord, but are not known by Him. They think what saves them is their action, but that seems not to be enough. The second house is anchored to the rock. It has the appearance of the first house in many ways, but beyond appearance it also has the rock, Jesus, as its foundation. So you can say all the right things, do the right things, but without the anchor of complete dependence on Jesus for salvation, it is not enough.
Take Away: My righteousness comes from Christ, not anything I do. I can look good on the outside, but that is not the point. I go back to the very first words Jesus said in this sermon, "Blessed are the poor in spirit, for theirs is the kingdom of heaven." My paraphrase would be, Satisfied are those who are gasping for breath, for them the kingdom has been prepared. Why? Because before we can understand our relationship to the kingdom and its ruler, we must see our brokenness, our need for a Savior. We must understand that what saves us is not our effort or obedience, but the blood f Christ, and that because of what Christ has done, this is what now drives our behavior. It is a subtle but critical difference. The life without a foundation is driven by a desire to be saved, and seeks to achieve that salvation by putting God in their debt, im a place where He is basically forced to grant them eternal life, the firm foundation recognizes that he is a sinner, and that there is nothing that he can do to obtain God's favor, in fact, he understands that even an attempt to do so is an insult to God. The firm foundation understands that salvation is a result of grace, and now lives a life that is a response to that understanding. Ephesians 2:8-10 states, For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast. For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them. Do I see what is going on here? That it is not my works that save me, it is grace. Yet for those who are saved by grace, there are works that are prepared for us to do. Not as an obligation, but as a response to grace. I still have a challenge in completely living in this manner, but it something that I strive for.