Jesus gave a clear explanation of what qualifies one to become a Christian in Matthew 16:24, "Then said Jesus unto his disciples, If any man will come after me, let him deny himself, and take up his cross, and follow me."
What does this mean? There are two basic steps in becoming a Christian.
1. The first step is to believe that Jesus died as a ransom for all mankind;
2. The next step is to decide to follow Jesus and learn how Jesus would have them live each day. It means that the person relies on Jesus and wants to live in righteous ways. Jesus said we should deny ourselves. This means that Christians give up their human will and do the will of our Heavenly Father, just as Jesus did. Hebrews 10:7, "Then said I
Perhaps you are thinking of someone who assumes that by saying the words, "I believe on Jesus," they become a Christian. Just saying the words doesn't qualify one to become a Christian, as just noted.
Perhaps you are thinking of someone who thinks that attendance in a church, or, giving money to a church, or participating in various church programs are enough to be considered a Christian in God's eyes. These alone would not qualify one as a Christian.
God wants total heart-devotion to Him. The way to live as a Christian – to live close to God and to Christ – is explained in Hebrews 11:6, "But without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he rewards those that diligently seek him." It takes effort, strength, and courage, and God will provide help to those who desire to be close to Him and to follow His son, Jesus.