Many Christians look at themselves to determine whether or not they are truly a believer. Many Christians think back to their experience of coming to Christ, when they first "made a decision" for Christ. They try to examine whether or not their decision at the time was genuine, they try to dissect their emotions or their sincerity or their level of knowledge. Many dig deep down inside themselves and examine their current doubts and struggles with various besetting sins. They compare their struggles and doubts with all the commands and/or descriptions in Scripture of what a mature and complete Christian ought to be and they attempt to weigh whether or not they measure up. Some Christians, probably most often those who have since become convinced of the Reformed perspective of things, look back to their original understanding of salvation and what God did for them in Christ and they can doubt if their original experience of coming to Christ was genuine because they didn't understand very much about the doctrines of grace or the way salvation is a work of God and not a joint effort between God and the sinner.
Doesn't Scripture teach us to examine ourselves to test and see if we really are in Christ? Yes, it does.
Examine yourselves, to see whether you are in the faith. Test yourselves. Or do you not realize this about yourselves, that Jesus Christ is in you?—unless indeed you fail to meet the test! (2 Cor. 13:5)Christians are to test themselves, to examine themselves. But what does this look like? Well, certainly an item well worth looking at is the fruit one's life is bearing. Do I manifest fruit and am I living a life in keeping with the gospel?
Only let your manner of life be worthy of the gospel of Christ, so that whether I come and see you or am absent, I may hear of you that you are standing firm in one spirit, with one mind striving side by side for the faith of the gospel... (Phil. 1:27)However, what Christians often do is a form of spiritual naval gazing, or what Douglas Wilson calls "morbid introspection". This is a process by which the Christian is always looking at themselves, always scrutinizing their own thoughts, desires and doubts. This seems at first like penitent and humble self examination, but it is really a self-centred practice. Rather than a humble and helpful growth-inducing discipline, perpetually staring at your heart through a spiritual microscope is really a prideful and self-centered false humility. This practice places me front and centre.
The true heart of faithful Christian self-examination is not the practice of looking at yourself but of looking to Christ. When you examine yourself or test yourself, look to see what it is that you are constantly turning to, constantly looking at, constantly trusting in, constantly holding to. If you are going through a season of doubt or if you are unsure of the genuineness of your faith because of your perpetual struggle with a particular sin, rather than trying to dissect your heart, look to the cross. Don't trust or doubt based on your feelings of trust or doubt. Turn your eyes to Christ, your one saviour in this world and your one hope for the world to come. Don't look at all the sin you still struggle with and place it in a mental scale to see if it outweighs the measure of your faith. Rather, take that sin and confess it to Christ. Don't spend all your time examining your inner thoughts and feelings and impressions. Rather, take this to Christ, place it at his feet, and look to Scripture's promises to you that Jesus has taken your sins, your guilt, your shame, even your doubts, and he has died for them. Then live in the knowledge that he rose again and sits at the Father's right hand interceding and advocating for you before a Father who knew you and determined to send his own Son to die for you before the world was formed.
The heart of true Christian self-examination is not constantly trying to look inside your own soul but rather looking at your way of life and your doctrine and from that asking yourself, what is it I am really trusting in? Whatever is inconsistent with a total and complete faith in Christ alone, confess and forsake. Then turn away from that self-examination and look to Christ. Rather than dwelling on the state of your own inner thoughts and feelings, dwell in the promises of God to you (Eph. 1:3-14) even as God's Spirit dwells within you. Rather than trying to test the genuineness of your confession of Christ based on your current feelings, look to what God has said about you in his Word and at your own baptism, where his triune name was placed upon you. Next time you are tempted to flick on the glaring spotlights of morbid self-examination and point them at your heart, reject your self-centred focus, remind yourself that you don't trust in your self anyway, pray to God to increase your faith, and then go rest in the pleasant shade of the cross.
Here is Doug Wilson on morbid introspection.