On page 137 of God is the Gospel, Piper introduces us to what Jonathan Edwards' calls, "the gratitude of hypocrites." You can read the entire Edwards quote in Piper's book; I will try to summarize what he is saying.
Could your thankfulness (gratitude) for the cross of Christ be idolatrous? Edwards and Piper answer "yes" if your gratitude for the cross is mainly from the faulty notion that the cross is about your worth. Edwards says, "They are pleased in the highest degree, in hearing how much God and Christ make of them. So that their joy is really a joy in themselves, and not in God." Ugh. Their joy is really a joy in themselves as opposed to a joy in God.
Let us check our gratitude for the cross to make sure it is not hypocritical!!