Do not be conformed to this world, but be transformed by the renewal of your mind, that by testing you may discern what is the will of God, what is good and acceptable and perfect. (ESV)
Have you ever known someone who seemed to always be putting on an act? Instead of just being their normal self they tried to act as if they were someone else. This kind of phony behavior is what the Apostle Paul is addressing here. We who belong to Jesus Christ have been transformed and regenerated by the power of God. We ought to act like it.
When Christians adapt their behavior to look like the world around them they are being completely inconsistent with who they are in Christ. This was the case with the people Paul was talking to in Rome. He is reminding them here that they are to not allow themselves to be like the world, but rather they are to be transformed.
So, how are we transformed? Paul says that this transformation happens by the renewal of our mind. We should all know how our mind is transformed. It is by the Word of God. The Word of God is always true. It is also always contrary to the values and beliefs of the world around us. We must fill our minds with the transforming and renewing eternal Word of God, the Bible.
As our minds are transformed by the renewing power of God's Word we will be able to engage our world and recognize those things that are contrary to the will of God. Discernment is such an important thing that a believer must do. Spurgeon said that discernment is not knowing the difference between right and wrong, it is the knowing the difference between right and almost right.
If you are a child of the King of Kings there is a need to act like it. As you fill your mind with God's Word you will be able to reject those things that are worldly and choose those things that are in line with God's perfect will.