We are told that the eleven disciples are in Galilee, where they see Jesus. Matthew notes that when they saw him they worshiped him. Then, Matthew adds, "but some doubted."
What did they doubt? The are with the resurrected Jesus; they surely are not doubting his resurrection. They are on the mountain where Jesus said he would met them; how could they doubt that he would make good his promises?
Jesus is on that mountain in order to send them out, into the world, to make disciples, baptize, and teach.
William Willimon, Methodist preacher and Bishop, asked in a sermon on this text if they doubted their ability to do the work that Jesus assigns them. He asked if we doubt our own ability to be Jesus' disciples charged with teaching, baptizing, and making disciples.
The Christian message is rooted in the self-sacrificial love of an incarnate God. The same God who did set aside the heavens in order to make His home with us now entrusts to us the telling of the story and the spreading of the good news.
Do not doubt - that you have been given this charge, nor that you are equal to the task.