From D.A. Carson’s “For the Love of God: Volume 1” January 6th devotion:
The first three sections of matthew 6 (which itself is the central chapter of the Sermon on the Mount) deal with three fundamental acts of piety in Judaism: giving to the needy (traditionally called “alms-giving”), prayer, and fasting (Matt. 6:1–18). The common link is striking: Jesus recognizes how easy it is for sinners to engage in worthy, philanthropic and even religious activities, less in order to do what is right than to be admired for doing what is right. If being thought generous is more important than being generous, if gaining a reputation for prayerfulness is more important to us than praying when no one but God is listening, if fasting is something in which we engage only if we can disingenuously talk about it, then these acts of piety become acts of impiety.
The fundamental way to check out how sound we are in each of these areas is to perform these acts so quietly that none but God knows we are doing them. So be generous, but tell no one what you are giving (6:1–4). Insist that even the recipients be silent. Pray far more in secret than you do in public (6:5–8). By all means, fast—but tell no one you are doing so (6:16–18). As for the middle item in these three traditional acts of piety, there is a further test: do not bother to ask your heavenly Father for forgiveness where you yourself are unwilling to forgive (6:14–15).
In each of these three traditional acts of piety, genuine Christian living is characterized by a simple yet profound desire to please God, and not by the ostentation that is in reality more interested in generating the impression among our peers that we are pleasing God.
The last two sections of the chapter continue this probing of our innermost motives. (1) In the first, Jesus tells us to store up treasure in heaven, for our hearts will inevitably pursue our treasure. What we ultimately value will tug at our “hearts”—our personalities, our dreams, our time, our imaginations, our inmost beings—and we will pursue it. That thing becomes our god. If what we value is merely material, our god is materialism. But if all we cherish most belongs to the eternal realm, then our whole being will pursue what is of transcendent significance. (2) In the second, Jesus tells us that a true and faithful relationship with God refuses to indulge in endless, needless fretting. We can trust God—his wisdom, his goodness, his providential ordering of things—even in this broken, evil world. Not to trust him betrays the pagan character of our hearts.
In short: seek first God’s kingdom and righteousness (6:33).