D. Martyn Lloyd-Jones said that Matthew 6 is one of the most uncomfortable chapters to read in all of Scripture because "it probes and examines and holds a mirror up before us, and it will not allow us to escape. There is no chapter which is more calculated to promote self-humbling and humiliation than this particular one." All-righty, then!
In Matthew 6:1, Jesus warns his followers to "be careful not to do your acts of righteousness before men, to be seen by them." Those last five words do not merely comprise a redundant modifier, but rather they hold the key to understanding the principle that Jesus is laying out.
If Jesus had ended his admonition with "before men," he would seem to be contradicting his earlier words, recorded in Matthew 5:16: "Let your light shine before men, that they may see your good deeds and praise your Father in heaven." But instead, he adds, "to be seen by them."
Mr. Peevie said that when he was in college, he and his peeps would go running. They'd make sure that their course took them across the center of campus so they could log some "face time"--an opportunity not only to be seen, but to have their healthful virtue admired by professors and peers alike.
Jesus says, essentially, don't do your good deeds with the motive of face time. Don't do them to be seen and admired by other people, and in fact, don't even let your left hand know what your right hand is doing! Don't dwell on them in your own mind; don't congratulate yourself for giving your left-over sandwich from lunch to the homeless guy at the train station. Don't think twice about what a good person you are because you gave up your seat on the El to a blue-hair.
This, for me, is way harder than it might seem on the surface. My favorite theologian, John R. W. Stott, wrote, “So subtle is the sinfulness of the heart that it is possible to take deliberate steps to keep our giving secret from men while simultaneously dwelling on it in our own minds in a spirit of self-congratulation.” Ouch. That is me to a T.
The positive expression of the principle that Jesus is getting at here is this: aim to please God only and always. This is, as Brother Lawrence put it, practicing the presence of God.