The greatest evidence that a person has genuinely become a follower of Christ is life-change.     1 John 2:3-6  states:  We know that we have come to know him if we obey his commands. 4 The man who says, “I know him,” but does not do what he commands is a liar, and the truth is not in him. 5 But if anyone obeys his word, God’s love is truly made complete in him. This is how we know we are in him: 6 Whoever claims to live in him must walk as Jesus did.   

Granted, we don’t walk (live) in perfection like Jesus, but we should walk in direction; walking (living) in the same manner as He lived.  In other words, there will be something different about our lives; such as a desire to obey the Word of God.

In Acts 19:18-20 we read:  18 Many of those who believed now came and openly confessed their evil deeds. 19 A number who had practiced sorcery brought their scrolls together and burned them publicly. When they calculated the value of the scrolls, the total came to fifty thousand drachmas.   20 In this way the word of the Lord spread widely and grew in power.

One reason we know their commitment to Christ was real is because of what we read in this passage.  1) They confessed their sin (their evil deeds) and didn’t try to hide this.  2) They were repentant which is evidenced by the burning of their books of magic, witchcraft and sorcery.  3)  Their repentance is further evidenced because in doing this it cost them a lot of money and how we respond with money is a great indicator of what’s really in our heart.

Matthew 3:8 AMP says:  Bring forth fruit that is consistent with repentance [let your lives prove your change of heart]

Matthew 4:17 AMP says:  From that time Jesus began to preach, crying out, Repent (change your mind for the better, heartily amend your ways, with abhorrence of your past sins), for the kingdom of heaven is at hand.

Paul said in Acts 20:21 NIV:  I have declared to both Jews and Greeks that they must turn to God in repentance and have faith in our Lord Jesus.

Repentance is not a word used much in our culture, but it was a big part of the vocabulary of the New Testament and genuine repentance is critical in the life of every follower of Christ.