Have you ever debated whether someone you know is a true Christian or not? You might argue that s/he has a strange belief or does a potentially immoral practice. In the end, only God knows, but that doesn’t mean that the He doesn’t provide guidance on what defines a Christian. 2 Corinthians 13 tells us to “examine yourselves to see whether you are in the faith; test yourselves”, so test your own life before considering your neighbors’.
In Matthew 7, Jesus tells His audience that not everyone who calls Him “Lord, Lord” will enter the kingdom of heaven. He goes on to say that those who hear and what He says are wise and build their life on a rock. The book of first John gives us more specific guidance on what makes a true Christian and includes three tests – the theological test, the moral test and the test of the Spirit.
The theological test is a test of belief or correct doctrine. 1 John requires us to believe that Jesus is the Christ (2:22), that He is God (or more specifically the Son of God) (4:15, 5:5, 10), that He came in the flesh as a man (4:2) and that the Father sent His only Son to make propitiation for our sins (9-10).
The moral test is a screen for our behavior. 1 John 3:6-10 requires us to stop sinning and practice righteousness. He tells us to keep God’s commandments (3:24a) and goes on to repeatedly emphasize loving our neighbor (3:10, 4:7-8, 16, 19-21).
The last of John’s tests is the Spirit’s witness within our heart (3:24, 4:13). He is the one who allows us to hear and obey the truth (2:20, 4:6).
Where do you stand with the three tests? Is your doctrine correct? Are you living out God’s commands? Do you have the testimony of the Spirit affirming that you’re in the family of God? If you fail any of these tests, you must seek to know God has He’s revealed Himself in the Scripture, seek His help to obey His commandments though trusting in Jesus to keep them perfectly and call to God for His’ Spirit to confirm that you’ve been born again.
Source: Grounded in the Gospel (Chapter 6) by JI Packer
James' Mirror - Christian Discipleship Guide 
The real test begins when God opens one’s mind to His truth. Once the Creator begins to reveal truth to a person, that individual bears a responsibility to act on it. If that person doesn’t show a willingness to live by what he or she has learned, God will add nothing more to his or her understanding. The person has shown that he really doesn’t want to do even what he has already learned.