Why did Isaac tremble when Esau came to him?

Les explains that Isaac suddenly breaks into trembling after Esau identifies himself as the firstborn. The first reason given is that Isaac was scared to death of what Esau might do; he may have realized Esau was the kind of man capable of hurting old Isaac for what he had done. Les acknowledges this is a thought, but points out Esau was a man destitute of faith. When people are destitute of faith, it affects their behavior, as seen in modern examples of teenagers with no sense of right and wrong, not comprehending violence or immorality. Faith changes that.