Thou shalt be dumb and not able to speak until the day that these things shall be performed, because thou believest not my words, which shall be fulfilled in their season. – Luke 1:20
Zacharias is a striking example of the ills a good man may have to suffer as the result of his unbelief. He was undoubtedly a believer. He is said, in the sixth verse, to have been righteous before God. No man ever obtained such a reputation except by faith. “The just shall live by faith.” No other righteousness than that which is faith is of any esteem in God’s account. Such was the righteousness of Abraham, and such was the righteousness of all the saints before the advent of our Redeemer. Such, too, has been the standard ever since. Zacharias evidently was a real believer. Yet for all that, when the angel appeared to him, and God gave him the promise of a son, he was amazed, bewildered, incredulous, and could not credit, but only question the announcement. “How shall I know that these things shall be?”
That he was well instructed in the Word of God is undeniable. He could not otherwise have discharged his duty, for the priest’s lips must keep knowledge, and he must teach men. Being proficient in the one, and competent for the other, ignorance offered him no excuse. Moreover, as a man of years, he was probably to be classed among the experienced saints of his time. He had borne the burden and heat of the day, and received proof upon proof of the abundant mercy of God. Now mark this. For any of us to doubt, who have been justified by faith is a shameful delinquency. For those to doubt who have, in addition to their first convictions, a thousand confirmations of the truth they have embraced, who are acquainted with the covenant and its rich inventory of promises, who are deeply taught in the things of God, for such to doubt involves a higher degree of guilt. I pray God that your conscience may be tenderly sensitive, and that you may be aroused to a sense of the dishonour you bring to Him by your faithlessness.~ C.H. Spurgeon
https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/3495.cfm