Zacharias had repented of his unbelief. Nevertheless he had had to live out the period of his discipline in a state of dumbness and probably also deafness.
Here is the best use of our tongues: to offer heartfelt praise and thanks to God. The worst thing he could have done was to be resentful about his punishment. His praise is a further way of acknowledging that he had been rightfully handled by God and had profited from it. All things work for the good of the believer, including their disciplines. Unbelief robs us of the power to praise God, and faith restores it by letting into our souls the glory of God. We are created to think carefully about all the wonderful works of God, and to respond in sincere worship.