Great question. I would say it is similiar to entering God's Kingdom. There is absolutely nothing we can do to earn our way in. We are completely dependent on Christ. Yet we still have an obligation to do whatever we can. We're to receive the sacraments, live holy lives, etc.
I liken the role of reason to our efforts. Human reason, though a gift from God, has absolutely no ability to comprehend God, yet we must use the reason we have to understand God as well as we can. So reason is beyond pale in comparison to faith, but serves a purpose even if only to get us reaching for the impossible.