so that you may approve what is excellent, and so be pure and blameless for the day of Christ, (ESV)
How good are you at spotting a phony? Years ago I worked at a bank as a head teller. We would have training on what a counterfeit bill would look like. There were certain things that we were to look for to make sure that the bill we were looking at was the genuine article. The best way to know what is counterfeit is to be intimately familiar with what is the true item.
In the previous verse Paul admonishes us to grow in knowledge and discernment. How do we grow in knowledge and discernment? By being intimately familiar with the true item. The Word of God is truth. There is no falsehood in it. We should regularly and faithfully be reading the Bible, familiarizing ourselves with every aspect of it's truth. In so doing we will know more and be able to tell the true from the almost true (discernment).
In our verse today, Paul tells us that the result of us growing in knowledge and discernment is so that we can approve what is excellent, and to be pure and blameless. I believe that one of the gravest concerns for the church at large today is that we are approving that which is mediocre at best and sinful at worst. Attention is not being paid to pursuing the truth.
"In the ancient world, dishonest pottery dealers filled cracks in their inferior products with wax before glazing and painting them, making worthless pots difficult to distinguish from expensive ones. The only way to avoid being defrauded was to hold the pot to the sun, making the wax-filled cracks obvious. Dealers marked their fine pottery that could withstand “sun testing” as sine cera—“without wax.”
- MacArthur Online Study Bible Notes
Can you spot a phony? If you do not know the Word of God the answer is no. You must dedicate yourself to the study of God's Word so that you can approve what is excellent.