In the Gospel Jesus asks us be honest about our praying, our fasting, our helping the poor. He wants us to be sincere, and not doing good things for show.
The word sincere really shows us how we must be. Back when people bought little images of their gods they often were cheated. The Latin word for wax was cherum, so the Latin for "without wax" was sincere.
Sometimes the molders of those images of the Roman gods, after pouring plaster into their molds, would find bubble holes here and there in their statuettes. When that happened the crooked statuette molders would fill the holes with wax, then paint over them.
A perfect image, without clay, was called sincere. You are sincere when you pray from the heart without just making a show of your piety.