Not everyone who responds to the gospel is necessarily a Christian.
The book of Acts tells the story of a sorcerer named Simon. His sorcery was a mixture of science and superstition, combining astrology, divination, and occult practices with mathematics and astronomy Simon had been deceiving people in Samaria with his acts of sorcery.
By the way, the devil can do miracles—lying wonders. Writing about the Antichrist, the apostle Paul said, "The coming of the lawless one is according to the working of Satan, with all power, signs, and lying wonders" (2 Thessalonians 2:9).
It may be that Simon's acts were devilish deeds, or they may have been sleights of hand. We don't really know exactly, but he was doing his thing, and everyone thought he was someone great.
Then Philip came to town and started preaching the gospel with the authentic power of God. A revival broke out, and people were abandoning Simon. Simon started realizing that Philip was bad for business, but there was no way that he could compete with him. So Simon faked a conversion. We know it wasn't a genuine conversion, because later, when Peter and John arrived, they were doing true miracles by the hand of God.
When Simon offered them money in an attempt to buy the power of the Holy Spirit, Peter rebuked him and said, "Your money perish with you, because you thought that the gift of God could be purchased with money! You have neither part nor portion in this matter, for your heart is not right in the sight of God" (Acts 8:20–21). Clearly this is not a description of a true believer.
You can't buy the power of God. There is no substitute for a genuine relationship with Jesus Christ.