Recently, a close friend asked me if a particular religion that claims to be Christian was truly Christian or was it a cult religion. First, let me tell you: I am not God's oracle; Jesus is. Instead of giving my friend a direct answer, I pointed him to certain scriptures so he could decide for himself. Kind of like the metaphor: Give a man a fish, and he will eat for a day; teach a man to fish, and you will feed him for a lifetime.* So, did the Bible and Jesus say anything about False Christianity? Yes: both Jesus and his book, the Bible, say a lot about other religions.

If you wish to know for yourself whether a religion is a Christian cult, it may help to know what the Bible says Christianity is. Christianity started with Jesus Christ, and it is important to understand that Christ is not a name; it is a title. Christ is the Greek translation of a Hebrew word in English spelled Messiah (See Strong’s Concordance # 4899). Christ means the “anointed one,” or the Priestly King typified by Gods covenant/promise to Abram, in which God said I will make you a father of many nations. (See Abraham's story, which begins in Genesis 12. Pay particular attention in Genesis 17 to the covenant/promise God makes to Abram. SEE also Romans 4, Galatians 3 and Hebrews 6.)

To learn how Jesus Christ's followers became known as Christians, read the Acts of the Apostles. The Acts of the Apostles follows the Gospel According to Luke, and the same person wrote both. Luke was not one of Jesus' original 12 Apostles. Luke was a Gentile physician who traveled with Paul and other Apostles after Jesus Christ's resurrection. Luke records that the locals in Antioch first called Jesus' disciples Christians because they behaved like Jesus.

Acts 11:25-26 KJV
[25] “Then departed Barnabas to Tarsus, for to seek Saul:” [26] “And when he had found him, he brought him unto Antioch. And it came to pass that they assembled with the church for a whole year and taught many people. And the disciples were called Christians first in Antioch.”

Then, according to the biblical narrative, a Christian is a follower (Disciple) who acts as Christ did. Jesus himself charged his disciples to love humankind as he did.

John 13:34-35
“A new commandment I give unto you, That ye love one another; as I have loved you, that ye also love one another. By this shall all men know that ye are my disciples, if ye have love one to another.”

Anyone can claim the title of Christian for themselves or a religion, so what is the dividing line for true Christianity? In other words, what makes one religion claiming the title of Christian true and another claiming the same false? All false religions, especially those false religions claiming Christ, start with a misconception about the deity of Jesus and who Jesus is.

Jesus sums it up nicely in that famous conversation with his disciples in Caesarea Philippi, when he asked his followers who they said he was. Peter boldly proclaims that Jesus is the Christ. A lot is going on during this event, and we can't cover it all here, so we'll cover the basic context. Jesus has been fulfilling the Law of Moses, Old Testament prophecies, and the Psalms concerning the Messiah/Christ. Despite all this evidence for Jesus’ divinity, the Jewish religious leaders at that time were vehemently teaching that Jesus was not the Christ and was not divine in any way. This is why Jesus tells his disciples to beware of the Jewish leaders' teaching. Later, after Jesus' resurrection, we read that some of these false teachers, who loved the praise of men, crept into the early Church and sought to corrupt its beliefs. Jude, in verse 4 of his short but powerful letter, referred to them as men who crept into the Church unawares.

Matthew 16:12-16 KJV
Then understood they how that he bade them not beware of the leaven of bread, but of the doctrine of the Pharisees and of the Sadducees. [13] When Jesus came into the coasts of Caesarea Philippi, he asked his disciples, saying, Whom do men say that I the Son of man am? [14] And they said, Some say that thou art John the Baptist: some, Elias; and others, Jeremias, or one of the prophets. [15] He saith unto them, But whom say ye that I am? [16] And Simon Peter answered and said, Thou art the Christ, the Son of the living God.

Anyone, of any religion, even if they claim to be Christian, who denies the nature and essence of Jesus Christ as revealed in the original documents of the Bible is an antichrist. Many false religions deny the nature of God in subtle ways, so research potential cults' beliefs well. The Epistles of 1 John and 2 John help define key things that false teachers deny about Jesus.
See: 1 John 2:18 - 22, 1 John 4:3, and 2 John 1:7

* Anne Isabella Thackeray Ritchie, in her 1885 novel Mrs. Dymond