"Not everyone who says to me, 'Lord, Lord,' will enter the kingdom of heaven, but only the one who does the will of my Father in heaven" (verse 21)
Although I never excelled in sports, I do remember at least one lesson from those traumatic days spent in middle school gym class. Whether the topic was tennis or basketball or golf or football, the teacher would include the importance of "following through" on the motion all the way to its completion. To interrupt or stop the action prematurely is to leave it incomplete and ineffective.
I believe Jesus makes the same point about our spiritual lives. It is essential that we not only hear and respond to His word - but that we see it all the way through. When we follow through on the Gospel, we stick to it, we persevere (ever wonder why there's so many scriptures encouraging us to keep going?), and we reap a bountiful harvest in the end. If we stop prematurely, we are living an incomplete Christian life. Victory awaits those who see it through and do all that our faith instructs us to.
Jesus completes the Sermon on the Mount in Matthew by driving home these ideas. To just call Jesus Lord without really living in a relationship with Him is not God's will for us - it will leave us on the outside of Heaven looking in. To just hear the words of Jesus and not actively put them into practice in our lives is like being a foolish man who suffers disaster by building on sand instead of rock.
When it comes to following Jesus, make sure you follow through!
"Everyone then who hears these words of mine and acts on them will be like a wise man who built his house on the rock" (verse 24).