From The Gospel of Matthew by Curtis Mitch and Edward Sri, commenting on Matthew 16:21-23:
It is significant that Jesus describes Peter as an obstacle. The Greek term refers to a “stumbling stone” that causes one to trip and fall. A contrast between this image and the rock in verse 18 is no doubt intended. The difference is one of grace verses nature. When Peter speaks what the Father has revealed to him, as he did in verse 16, he is the sturdy foundation stone that keeps the forces of darkness at bay. But when the same Peter speaks from the standpoint of weak human nature apart from divine assistance, he is a stone that causes others to stumble.