Unfortunately, there isn't a simple answer to that question. Christians in the Bible were followers of Jesus who became known as Christians. These first century Christians were the original, true Christians. Unfortunately, the Bible states that a great apostasy would occur and false teachings and beliefs based on pagan religions would enter into the Christian congregation. Jesus said that this would occur and illustrated this with the wheat and the weeds illustration. At Matthew 13;24-30&36-42 Jesus said that he would plant wheat (his followers who became Christians) and while men were sleeping (after the apostles fell asleep in death), Satan would plant weeds that would quickly overtake the wheat Jesus planted and become the dominant type of Christian. Jesus said he would not stop this and would only take action during a time he calls "the conclusion of the system of things." The action he would take during "the conclusion of the system of things" would be to separate the wheat from the weeds and to put them into two groups - false Christians represented by weeds and true Christians represented by wheat. The weeds would eventually be permanently destroyed at Armageddon and the wheat would be the ones who survive Armageddon.
When did the weeds begin to be planted? False teachings started to make their way into the Christian congregation in the first century. The following scriptures are warnings showing this (Matthew 7:15;2 Thessalonians 2:3;2 Peter 2:3;1 Timothy 4:1; 2 Timothy 4:3,4). However, Jesus said that the weeds would be planted "while mean were sleeping." The men were the apostles. While they were alive they were able to stop the weeds from being planted but Paul said at Act 20:29,30 that after his death "oppressive wolves" would enter in the congregation and "speak twisted things." The last apostle, John, died at the end of the first century. After his death and at the beginning of the second century, it's a historical fact that the Christians decided to try to make more converts by appealing to the educated Romans who were heavily influenced by Greek philosophy. They did this by combining Greek philosophy with Christian beliefs. For example, Jesus was compared to Perseus; and Mary's conception to that of Perseus' mother, Danae, who was said to be also a virgin. Another example is the term soul. The Hebrew word for soul nephesh and the Greek word psykhe are used in the Bible to refer to mortal, living creatures both human and animal (Genesis 1:24,2:7,1 Corinthians 15:45)but these second century Christians linked the soul to the belief popularized by Plato which is that the soul is separate from the body, immortal and invisible. Adopting pagan religious celebrations and festivals of both the Greeks and Romans and making them Christian holidays also started back in the first century and continues down to our day with just about all of the holidays celebrated by Christians. This type of hybrid Christianity (weeds) started in the second century after the apostles died (men where sleeping) and became the dominant type of Christianity for centuries. All of the major Christian religions whether Catholic or Protestant are based off of this type of Christianity. The Bible makes it clear that God rejects religions that are a mixture of Christian and pagan beliefs(John 4:23, 24, Colosians 2:8, Hebrews 13:9). Jesus said in the wheat and the weeds illustration that during the conclusion of the system of things he would make the wheat clearly evident. Matthew chapter 24 and 25 and 1 Timothy 3:1-5 are two of many scriptures that show we are deep in the last days. Jesus has already separated the two groups. Unfortunately, since there are now so many beliefs and people aren't taught the truth about the history behind where their religion came from, it's hard to tell which one is right. However, Jesus did give one identifying mark of his true disciples.
At John 13:35 Jesus told his disciples that the love they showed among themselves would be that identifying mark. That love would transcend race, nationality and socioeconomic class. The love they showed among themselves would be international. So there would be no national divisions among spiritual brothers and sisters from the same religion where if their countries went to war they would take their countries side and kill each other which has happened with Christendom all throughout the 20 century and would still happen today. Would Jesus identify members of those religions who supported or participated in wars that killed spiritual brothers and sisters of the same faith as having love among themselves? There's only one Christian religion who has never picked up arms and participated in the wars of the 20th century, refuses to join the military so there won't be any chance of killing their "neighbor" or another fellow spiritual brother or sister. They feel that when God looks down at the many Christian groups offering their worship to Him he can see that they are different and truly do have international love amongst themselves. And that they have removed any and all pagan practices and beliefs that the majority of the Christian practice and believe.