In Rom. 12:3, Paul doesn’t just leave his instruction with the prohibited thinking, but also supplies the prescribed way of thinking.  Instead of self-exaltation, all Christians are to think sensibly or perhaps with sanity.  This is the word that is used to describe the demoniac in Mark 5 after Jesus cast all the demons out of him. The wording in that passage indicates that after Jesus’ miracle, he was in his right mind.  Thus, when we are thinking rightly of ourselves, then we are in our right minds.  It follows then that if there is any self-exaltation, then we are out of our minds.
Notice the end of v.3 that provides great help to lead us to sensible thinking—God has distributed to each a measure of faith.  At the outset here, it’s crucial to point out that this faith is not referring to saving or justifying faith.  There are not different amounts of faith that God grants, which then people employ to become justified before him.  If Paul wanted to highlight the general truth that God grants faith to believe in Him, then using the word, measure, is highly unnecessary and confusing.  However, if he wanted to highlight why some seem to have so much impact using their gift as a way of invoking sober mindedness and humility, then this statement makes so much more sense.  This measure of faith, then, is referring to another gift from God, which accompanies the supernatural gifting or ability to edify and serve others.  In other words, God not only gives the spiritual grace-gift, but also the faith to use the gift.
The way this is worded ensures that God gets all the glory and credit for the usage of the gift.  Therefore, the point isn’t to try to figure out how much faith God has granted and then cap yourself off once you think you’ve hit the mark. No!  God hasn’t revealed the amount and therefore he doesn’t want us to know it.  He desires that we use our gifts he has given us to the maximum potential and that’s why he’s supplied the fuel we would need to be faithful with what he has given us!  Furthermore, this measure of faith description answers the question why someone can have the same gift and use it faithfully, but yet not have the impact and influence that another person has.  Since God has given everything we have and need to be useful in the body, it would be irrational to exalt oneself for simply using what God has given.