Ephes. 4:17-24 (ESV)
Now this I say and testify in the Lord, that you must no longer walk as the Gentiles do, in the futility of their minds.  They are darkened in their understanding, alienated from the life of God because of the ignorance that is in them, due to their hardness of heart.  They have become callous and have given themselves up to sensuality, greedy to practice every kind of impurity.  But that is not the way you learned Christ!—  assuming that you have heard about him and were taught in him, as the truth is in Jesus,  to put off your old self, which belongs to your former manner of life and is corrupt through deceitful desires,  and to be renewed in the spirit of your minds,  and to put on the new self, created after the likeness of God in true righteousness and holiness.
Hardness of heart often blinds the Gentiles, here unbelievers, to the true nature of God. However, sometimes we as Christians do this too. We have preconceived notions of who God is and we cherish these little idols in our heads. This is why Paul writes to Christians telling them not to walk as Gentiles, because our Old Adam, our old self, is still with us and would lead us astray. The only cure for this is the word of God that is the truth in Jesus, the way we learned Christ. In Jesus Christ we see God for who he is, he reveals himself to us. He reveals himself as a redeemer, in mercy on the Cross forgiving the sins of the world. So we let him shape and mold us into his image, rather than trying to shape and mold him into our image. We do this by reading the Bible and learning from it, discarding our notions of right and wrong, of who God might be, and letting him show us right and wrong, and who