Anti-rationalism and obediance

I have just returned from another khutba, where the khateeb could not resist denouncing rationalism in religion. The topic of the khutba was pilgrimage, Makka, the Ka`ba, etc. Then, towards the end, the khateeb digressed on the story of Abraham leaving his infant son Ismael and wife Hagar in the middle of arid desert. He said that this is the example we have to follow -- so that we may do what is asked of us by God, rather than subject everything to deductive analysis.

The question, again, is how one know what is asked of him! Indeed, the Qur'an contains the answer to those who claim that they were merely fulfilling God's command:

وَإِذَا فَعَلُوا فَاحِشَةً قَالُوا وَجَدْنَا عَلَيْهَا آبَاءَنَا وَاللَّهُ أَمَرَنَا بِهَا
قُلْ إِنَّ اللَّهَ لَا يَأْمُرُ بِالْفَحْشَاءِ أَتَقُولُونَ عَلَى اللَّهِ مَا لَا تَعْلَمُونَ

When they commit a blameworthy act, they say: "We found our fathers doing so"; and "Allah commanded us thus;" say: "Allah never commands what is blameworthy: Do you say of Allah what you know not?"

Were it not for the rational faculty, it would be impossible to know what is righteous and what is sinful. The basis of faith in Allah is:

إِنَّ اللَّهَ يَأْمُرُ بِالْعَدْلِ وَالْإِحْسَانِ وَإِيتَاءِ ذِي الْقُرْبَى
وَيَنْهَى عَنِ الْفَحْشَاءِ وَالْمُنْكَرِ وَالْبَغْيِ يَعِظُكُمْ لَعَلَّكُمْ تَذَكَّرُونَ

Allah commands justice, beautiful deeds, and generosity to kin,
and He forbids blameworthy deeds and transgression:
He instructs you, that ye may remember and reflect.

That is how we know that God's orders must be obeyed: so that we may do good and avoid evil. There is no substitute for the rational faculty in determining what is good and evil. Scripture helps us to train and refine our senses of good and evil, but in the end, it is out responsibility to make the inference -- both deductively and inductively -- to the best of our rational abilities.