This is a quote from John MacArthur's article titled "Defining Discernment"
“The key to living an uncompromising life lies in one's ability to exercise discernment in every area of his or her life. For example, failure to distinguish between truth and error leaves the Christian subject to all manner of false teaching. False teaching then leads to an unbiblical mindset, which results in unfruitful and disobedient living-a certain recipe for compromise.
Unfortunately, discernment is an area where most Christians stumble. They exhibit little ability to measure the things they are taught against the infallible standard of God's Word, and they unwittingly engage in all kinds of unbiblical decision-making and behavior. In short, they are not armed to take a decidedly biblical stand against the onslaught of unbiblical thinking and attitudes that face them throughout their day.
“Discernment intersects the Christian life at every point. And God's Word provides us with the needed discernment about every issue of life. According to Peter, God ‘has granted to us everything pertaining to life and godliness, through the true knowledge of Him who called us by His own glory and excellence’ (2 Peter 1:3). You see, it is through the ‘true knowledge of Him,’ that we have been given everything we need to live a Christian life in this fallen world. And how else do we have true knowledge of God but through the pages of His Word, the Bible? In fact, Peter goes on to say that such knowledge comes through God's granting ‘to us His precious and magnificent promises’ (2 Peter 1:4).
“Discernment -- the ability to think biblically about all areas of life -- is indispensable to an uncompromising life. It is incumbent upon the Christian to seize upon the discernment that God has provided for in His precious truth! Without it, Christians are at risk of being ‘tossed here and there by waves, and carried about by every wind of doctrine’ (Ephesians 4:14).”