One of Christ’s most striking teaching tools is hyperbole: “If your hand offend you, cut it off”; “whoever says ‘you fool’ is liable to Gehenna”; etc. The explanation of hyperbole is frequently left at mere deflation of the sense: we explain that people shouldn’t cut their hands off, and that not every use of bad language is a mortal sin. The difficulty with leaving the explanation at this is it doesn’t explain why the exaggeration was used. The whole explanation of hyperbole can’t be that the one using it meant less than he said. If this were all he meant to do, he would have just said less himself.
One suggestion would be to say that Christ uses hyperbole to convey the idea of an unreachable limit: “cut off your hand” in this case means to say “you will never cease to have something that is in danger of leading you to sin” or “you will never be able to totally remove all that could lead you to any sin” or more optimistically “the work of perfecting yourself must be continual”. This idea of hyperbole signifying the unreachable is suggested by Christ’s monetary examples: when he speaks of the debt of a servant which clearly signifies the debt of sin, he calls it a debt of 10,000 talents. One talent was 6000 denarii, and one denarius was a days wages. The implication is that no one could ever pay off his debt of sin.
The doctrine of the Gospel is the doctrine of perfection, as St. Thomas says, and in some respects perfection is an infinitely approachable limit: “be perfect, as your Father in heaven is perfect”. Christ’s teaching is better than any other at laying out this infinite approach to perfection with the greatest clarity and the greatest economy of words.