Here’s my shot:
Faith is required as a proof of love of God.
Faith was not required of Adam, who clearly communed with God, and yet love was required of him.
The relationship between God and man is one of love, not slavery or subservience. But love requires that one give oneself to the needs of the other. (It is not a feeling,since we cannot be commanded to feel and yet we are commanded to love.) God has no need to be filled. Without some tangible evidence, man could not be credibly understood to love God. God, therefore, gave man a means of demonstrating his love.
In the Garden, the existence of the untouched tree of life (in other words, the refraining of committing a specific, insignificant act) was proof of the existence of love between God and Adam. Eating the fruit of the tree was unnecessary in the abundance of Eden. Complying with the command not to eat that one fruit by virtue of its insignificance demonstrated a willing desire on the part of Adam to please God, and demonstrated nothing else.
But Adam consumed the fruit, and it was hence impossible to prove or to know by virtue of refraining from an act his was a love of God and not subservience to him. It is not enough to say he would not do it again, since refraining from all future acts would not repair the one, demonstrating only a fear of consequence but no longer love. The test was destroyed for all time.
Once the refraining from an insignificant act lost its demonstrative power, God is left to require of man an affirmative act - faith.