There are a lot of lessons to learn from yesterday's Mass readings. The gospel was from Luke 18:9-14 and was the story of the Pharisee who thanked God for not being a sinner like the nearby tax collector; meanwhile the tax collector acknowledged his sinfulness and begged God for forgiveness. Jesus said that "the latter went home justified, not the former".
This parable shows us several important lessons that often get misunderstood. First, I've often heard people--particularly Catholics--say that "as long as I'm a reasonably good person then I'll go to heaven". The idea is if we hit some sort of minimum requirement of "goodness" then we'll make the cut. But look at this Pharisee. He did all the right stuff, he fasted, tithed, and went through all the motions required and as such he was certain that God would reward his efforts. Jesus tells us that he was in for disappointment. Not a single one of us is "good enough" to deserve heaven. That's not how it works. The only way into heaven is to accept the grace, forgiveness, and salvation offered by God through Christ. That's the example we see in the tax collector.
Second, Jesus directs this parable specifically to those "convinced of their own righteousness and despised everyone else". The second lesson then is to... well... not be one of these people! Sounds obvious but we probably all know someone who (or are ourselves) is so sure of their salvation--so sure that they have it all figured out--that they can relax a bit. I know I've had plenty of moments in my life where I've given into the notion that "I'm doing pretty well, I can back it off a bit" or "Now that I've got this God-thing figured out I can focus on other things a little more". I'm well acquainted with the decent down the slippery slope of spiritual haughtiness to wondering why God felt so distant "all of a sudden". Spiritual pride is one of the worst cancers a person can get; and Jesus rarely passes up an opportunity to warn us against it.