13 Even Simon himself believed; and after being baptized, he continued on with Philip, and as he observed signs and great miracles taking place, he was constantly amazed.[Acts of the Apostles 8:13]
20 But Peter said to him, “May your silver perish with you, because you thought you could obtain the gift of God with money! 21 You have no part or portion in this matter, for your heart is not right before God. 22 Therefore repent of this wickedness of yours, and pray the Lord that, if possible, the intention of your heart may be forgiven you. 23 For I see that you are in the gall of bitterness and in the bondage of iniquity.”[Acts of the Apostles 8]
Simon Magus was said to have believed and was even baptized. Yet, later on, his wicked heart was exposed.
Then many believers left the Christ at the very end of John 6. The Christ asked His disciples, ”You do not want to go away also, do you?”[vs 67] Peter replied, ”Lord, to whom shall we go? You have words of eternal life. We have believed and have come to know that You are the Holy One of God.”[vss 68,69]