“No good tree bears bad fruit, nor again does a bad tree bear good fruit; for each tree is known by its own fruit. Figs are not gathered from thorns, nor are grapes picked from a bramble bush. The good person out of the good treasure of the heart produces good, and the evil person out of evil treasure produces evil; for it is out of the abundance of the heart that the mouth speaks.
“Why do you call me ‘Lord, Lord,’ and do not do what I tell you? I will show you what someone is like who comes to me, hears my words, and acts on them. That one is like a man building a house, who dug deeply and laid the foundation on rock; when a flood arose, the river burst against that house but could not shake it, because it had been well built. But the one who hears and does not act is like a man who built a house on the ground without a foundation. When the river burst against it, immediately it fell, and great was the ruin of that house.”
Why, if salvation is through faith in Christ and only in faith, does Jesus make such a big deal about what we do and say? What is the purpose of the Sermon on the Mount? Jesus ends his dialogue with two answers to those questions.
The first answer is simple enough. If we don’t act Godly, then we don’t have Godly hearts. Our words and deeds are the reflections of what is in our hearts. Jesus’s description of “abundance of the heart” gives me a mental image of a flooding lake spilling over the sides of a dam. I imagine my heart filled to the brim with emotions until there isn’t room for more, and they come spilling out as words. If we really and truly love God, our words and deeds should naturally reflect that, and should naturally change. But we are still sinners, so maybe sometimes we have to change our behaviors first in hopes that our hearts will follow suit.
The second answer is harder to understand, though I think it relates to the first. Following Jesus’s command helps us to build a foundation for our faith. When we fail to change the way we interact with (and view) the world by taking the words of God into our hearts and actions, at the first sign of difficulty our faith will come crashing down around us. We know what God says, but we never truly experience His glory and love. So we forget why we believed in the first place. But when we do follow Jesus’s commands, we feel closer to God. And that builds a foundation for a relationship that can weather even the worst situations.