Albert Barnes Commentary Luke 1:20

"And behold, thou shalt be silent and not able to speak, until the day that these things shall come to pass, because thou believedst not my words, which shall be fulfilled in their season." — Luke 1:20 (ASV)

Because you do not believe, this was both a sign and a judgment—a sign that he had come from God and that it would be fulfilled; and a judgment for not believing what he had said. There is no sin in the sight of God more aggravated than unbelief. When GOD speaks, people should believe; nor can anyone who 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 believe what he says.