Jesus begins Matthew 6 with a warning against the spiritual sin of self-righteous hypocrisy. Using the examples of those who may seek the praise of men (instead of the favor of God) through their public alms giving, praying, and fasting, Jesus gives us the sharp warning to, "Beware." Instead of devotion to God being the goal of their alms giving, praying, and fasting, they desire to be considered religious by their viewers for doing so. They wanted to be known as, what we may call today, as "Super-Christians".
The Greek word used here for "beware" is prosechete. It simply means to "be on guard". As I've studied this passage, I have come to understand that this is not simply an indictment against the Pharisees. It is a warning for all believers. All of us need to beware of this sin. We have to be on guard against the desire to be known as the "most spiritual Christian" in our church. We have to guard against the tendency to want to be known as the most liberal giver, the most powerful prayer, and/or the most committed to fasting. And in this warning, we are reminded of why we should ultimately do these things--to show devotion to the Lord!
We should strive to honor God in all of these things, understanding that we will be rewarded for our devotion and faithfulness to God. When our own selfish ambitions and motives muddy our clarity to the point that we begin doing things for our own recognition, we must hear the words of Christ. We must be reminded that there are no frequent flyer self-righteous awards in heaven. In fact, if we do these things for our own recognition (whether it is preaching a sermon, singing a special on Sunday morning, teaching a class, saying a blessing, etc.), we will not be rewarded by the Father.
The first will be last. The last will be first. God never sees what we do without also being able to see the motives behind our doings. Keep the right perspective. Be on guard against self-righteous hypocrisy.
Keep glorifying God. Keep your eyes fixed on Jesus. He rewards those who earnestly, diligently seek Him.
And remember, there are no self-righteouss, Super-Christians in Heaven. There are only forgiven sinners that have been saved by the grace of God through the shed blood of Jesus Christ.