Faith must trample under foot all reason, sense, and understanding.
It would be simplistic to take this quote of Luther's at face value. After all, his collected writings amount to 80 fat volumes in English - lectures, sermons, treatises, occasional writings, even the famed "Table Talk" and hymns. Yet, Luther's opposition to "reason" (he actually called it a whore) was based on the over-reliance on technical reason in the Roman Catholic Church. Without belaboring the historical point, Roman Catholic theology at the time of the Reformation was a baroque castle in the air, bereft (as Luther saw it) of the simple message of salvation by grace through faith, as contained in the Scriptures of the Old and New Testaments.
Yet, there is also something correct in this formulation. At the end of the day, after all the theological reflection, Biblical exegesis, and consideration of the history of the Church, if our faith is not a living, breathing thing concerning our encounters with our brothers and sisters, living out love for all creation, it is really not faith at all. This is not to denigrate the life of the mind, or its place in the life of the faithful Christian. It is only to put it in its proper place. Being seized by grace means all of our life is now under that marvelous law of love and freedom. Our faith must seek understanding, but it must be a real, living faith to be understood.
It might just be, for example, that there are all sorts of sound reasons, neatly thought out and considered, as to why one should not become an ordained clergy person, say, or a missionary. A successful career, a happy family, comfort and a good reputation. Yet, if one is responding in faith to the call of God, all those reasons "why not" end up being trampled under foot. I have seen it far too often to deny the simple reality that faith takes all the ways we deny it, laughs at them, balls them up, and throws them away.
Part of the freedom of being a Christian includes the freedom from the dictatorship of all the ways we tell ourselves we need not believe, need not follow, need not live in love toward all creation. A living faith is a faith that is always moving, certainly always reflecting, but also never submitting itself to the dictates of any set of rules that do not begin, "First, love God with all your heart and all your mind and all your soul and all your strength, and your neighbor as yourself. In this is all the Law and Prophets."