But behold, you will be mute and not able to speak until the day these things take place, because you did not believe my words which will be fulfilled in their own time. – Luke 1:20

If we believe not, yet He abideth faithful; He cannot deny Himself. – 2 Timothy 2:13

Now Elizabeth’s full time came for her to be delivered, and she brought forth a son. – Luke 1:57

His promise still stands. God does not take advantage of our unbelief to cry off and say, “I will give thee no blessings because thou doubtest Me”-no, but having said it, He does it and His Word does not return unto Him void. Even the trembling, doubting children, though they get the rod, get the blessing too. Very painful, indeed, was Zacharias’ chastisement. One would not like to be deaf and dumb for a day; but to be deaf and dumb for the space of nine months must have been a very painful trial to this man. Moreover, he could not bless the people; he could not speak a word; he could not instruct the people; he was useless for that part of the priest’s work; and when the song went up within the hallowed walls of the temple, he could not hear it.

Fatherly chastisements come upon us as the result of our unbelief. Cry to God. Our God deserves better homage of us than a weak, attenuated faith can render Him. He deserves to be trusted with such confidence as a child gives his parent. Ask Him to increase your faith. And you who have faith, oh! keep it jealously, exercise it habitually; pray to the Lord to preserve it. Never begin to walk according to the sight of the eyes. Confer not with flesh and blood. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/3495.cfm