Albert Barnes on Luke 1:20

Because thou believest not (z) , &c. This was both a sign and a judgment-- a sign that he had come from God, and that the thing would be fulfilled; and a judgment for not giving credit to what he had said. There is no sin in the sight of God more aggravated than unbelief. When GOD speaks, man should believe; nor can he that will not believe escape punishment. God speaks only truth, and we should believe him. God speaks only what is for our good, and it is right that we should suffer if we do not credit what he says.

(z) Eze 3:26