In the 1940s Harry Blamires wrote, ‘There is no longer a Christian mind. There is still, of course, a Christian ethic, a Christian practice and a Christian spirituality’ (from: The Christian Mind: how should a Christian think?).  He was, and is, right.  Likewise, John Stott encouraged Christians to 'think Christianly'.  But the idea is still remote to many who claim to follow Christ, even though Peter wrote about it two millennia ago.  He uses the term 'wholesome thinking' which means the kind of thinking that comes from considering what is true of God and the gospel, and which leads to integrity of belief and action.  It stems from ‘sound teaching’ which the apostles taught and is recorded for us in the Bible (2 Timothy 1:13)
 
It is quite possible to have Christian ideas and behaviour, and even participate in Christian forms of worship, without having thought about any of it.  It is just a routine or ritual.  Such believers are at serious risk of being seduced by persuasive spiritual entrepreneurs, or disillusioned when life is hard.  By contrast, Peter emphasises the need to understand the basis of our faith in Christ, in order to be well rooted and stable, sure footed and able.  Bringing something to mind, in order to take action, is a choice.  Peter’s concern was that his readers would choose to remember what the Bible says, and do it, and not ignore the truth (Luke 6:46-49).

Paul also knew the importance of being absolutely sure about his faith as he faced suffering (2 Timothy 1:12).  Unless believers understand clearly why they are safe in Christ, they will feel unsafe; and their doubts will make them vulnerable to their own doubts or the solutions offered by false teachers.  So, any Christian ministry which does not stimulate people to think about God's Word and its implications, is dangerously defective; even though it may have many adherents.
 
The way to start thinking Christianly is to understand what the Bible says.  That is the only satisfactory way to know what is true.  God still speaks through the text, written by prophets and apostles but all inspired by the Holy Spirit.  The more we immerse ourselves in the Scriptures, the more our minds become calibrated to understand what pleases or offends the Lord (2 Timothy 3:16-17).  Then we can use Christ-calibrated minds to understand why and how the world around us is so dysfunctional, and why the gospel is so essential.  Those who think Christianly will live like Christ (because that is a radical choice which is impossible without a serious thought-through decision).  And only those whose mind is Scripture-tuned will dare to move from a comfort zone to mission.  All the great missionary pioneers, whose legacy is now seen world-wide, were people of the Book.  And interestingly, that is what some people still call us.  So, let's live up to our reputation!