Truly Knowing God
In Matthew 7:22-23, Jesus tells us that on the other side of eternity there will be people who thought they were on good terms with God (and if we saw them doing the things they were doing, we probably would assume the same). They prophesied, cast out demons, and performed miracles all in the name of Jesus. That's a pretty good resume...
But Jesus is more concerned about their hearts than merely their actions. They did good things, and they knew about God, but apparently they missed truly knowing God Himself: intimately, personally, lovingly.
If you're in Christ, the most true thing about you is that you are a forgiven, adopted son or daughter of God.
Doing good things for God is great, but let's not get so busy working doing good things that we miss the best thing: knowing and loving God.