It is impossible to serve G-d, both in relations with G-d and with one’s fellow man, without submitting to the Revelation of His Will. Those who say they will figure out for themselves, using their limited minds, what G-d wants, instead of seeking His perfect Revelation, err greatly. Likewise, all people who treat their relationship with G-d as something that they do on their own fallible terms instead of on His terms, are bound to fail–nay, they have already failed–and will inevitably be lacking in that relationship.
Put simply, if my boss tells me to do a and I do b, I will be fired regardless of my intentions. Similarly, if my wife tells me that she likes flowers and not chocolates, and I bring her chocolates because that’s what I like, she will be displeased regardless of all my good intentions.
The Torah is endlessly vast and contains guidance in all aspects of life concerning how one should behave in any situation. So when it comes to any question, the only question one should ask oneself–or, in the many cases in which one lacks the sufficient knowledge, one’s rabbi–is: What does G-d say in His timeless Torah, the manual for my life today, concerning how I should act?
But no matter how clever and well-educated one may be (including education in Torah), asking oneself in a G-dless, secular way “what do I think” will inevitably lead to selfish, wrong conclusions to a lesser or greater extent, because whenever one chooses to leave the G-d of Truth out of the picture, truth and objectivity are impossible.