"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."
Even though Christians are eternally secure in Christ, we still must live in this present age. It is one still filled with multiple temptations and awful evil. Christians must fight against its pressure to mold us into its image.
What is the key to keep from being molded into the image of this present age? It's the renewal of our minds. We are what we think. Whatever we focus our minds on is what we will eventually become.
Then, as our minds are renewed, we are able to discern what is good and evil. We are able to know God's will and obey it.
This "testing" that comes from living in the world is a good thing. How can an athlete know if he's successful unless he plays against an opponent? This testing of living in a fallen world allows us to know if our faith is real and our minds have been truly transformed.
If you want your life to be new and faithful to Christ, look at your thought life. What you think eventually become actions.
Do you have the mind of Christ? Do you think as he would will you to think?
The faithful Christian life begins there.