Malachi 3:1-4 (There is a day promised when God will save the people from their enemies and give them a place to live free in the world. But it sounds almost like bad news instead of good, when it becomes clear what kind of deep cleansing and purification of the people this will involve.)
Luke 1:67-79 (When the angel told old Zechariah that he and his equally old wife would have a son who would pave the way for Jesus, Zechariah couldn't believe it and God silenced his doubting oice by shutting his mouth. When John is finally born, Zechariah is opened to holy Spirit in a new way, and bursts into a song of praise to God's good will and promises fulfilled.)
Zechariah didn't think he had it in him. Nor did he think his wife had it in her. When the angel Gabriel (Gabriel of all angels!) told Zechariah that he and his wife would have a son who would pave the way for God's messiah to come to the world, his first thought was, "No way we can do that!" He was more mindful of their limitations and the barrenness of their history, than he was of the power of God.
Rather than argue with him, though, God simply shut Zechariah up and went about the business of preparing and doing what was promised, using and helping Zechariah and wife to accomplish something they had not been able to, to that point in their lives.
God doesn't need us always to believe right from the start. But God does require our willingness to take part -- and sometimes a hard silence about our doubts and misgivings while we are doing our part.
And then, when what was promised comes to be, willingness on our part to give credit where credit really is due, is always a beautiful thing.