In my years as a spiritual director, I have encountered two individuals whose main issue was the question of certainty. Both expressed not only a deep need to know beyond any doubt the answers to their questions about the nature of God and the role of a divine force in human activity, but also a determination to pursue their questions until the answers became clear. For one, the quest emphasized studying and discussing theological texts one after another; for the other, the effort had devolved into repetitive spiritual direction sessions in which the same questions arose over and over.
I am grateful to both individuals for making clearer the role of faith in spirituality. Both of these seekers had identified the mystery that surrounds the infinite, but instead of being awestruck and reverent, they were afflicted with a terrible need to make the mystery disappear, to conquer it, and to reduce it to a size they could grasp. They were rejecting the human side of the spiritual relationship: acceptance of ambiguity and belief that there is a balance of what God brings (grace) and what we bring (faith).
This may not be the full explanation of what the Bible means by the statement that we must change and become like little children, but it might be part of it. Little children can have no certainty about anything outside of the faith they have in their parents. Likewise, in our spiritual relationship, our only certainty is the faith we bring to it.