It really is amazing how often sceptics and heretics line up. One would almost think the same devil is hiding behind both mindsets (hint hint). One such similarity is in the definition of Christianity. A heretic will get upset if you imply they are not Christian, and an atheist will accuse you of the no true Scotsman fallacy for the same thing. Their reasoning is identical: "They believe in Jesus".
But the Bible tells us a different story. It predicts, and pre-emptively refutes, the rise of a number of pseudo-Christian religions, both in its own time, and in the future. Paul has this to say: "But I fear, lest somehow, as the serpent deceived Eve by his craftiness, so your minds may be corrupted from the simplicity that is in Christ. For if he who comes preaches another Jesus whom we have not preached, or if you receive a different spirit which you have not received, or a different gospel which you have not accepted—you may well put up with it!"
Clearly, then, there are three essential elements for Christianity, and in order to deceive people, Satan creates counterfeits. These three elements are the Jesus whom Paul and the Apostles preached, the spirit the Church received, and the Gospel they accepted (as was preached by the Apostles).
Many religions exist which call themselves Christian, yet lack one or more of these elements. To say these religions are not Christians is like saying a lizard is not a salamander, a moth is not a butterfly, or a snake is not a caecilian. It's not about "no true Scotsman", it's about "that so-called Scotsman never set foot outside of China". If a so-called Christian does not know the real Jesus, does not have the Holy Spirit in his heart, and abides by any other gospel, he is as bound for Hell as an unbeliever (Galatians 1:8).
This problem persists in denominations and individuals alike. Some religions teach that Jesus never had a real body, and was a phantom. Some religions are excessively violent. Some religions add works to grace as a requirement for salvation. These are all examples of things the devil can change about Christianity to make a fake Church, and present it as real Christianity, both to the world, and to those who fall for the lie.
So what's the solution? Much like I have done in this article, the solution is to study the Bible. It helps that most fake Churches either don't use the Bible, or discourage independent study of it. Jehovah's Witnesses, for example, teach that their Church is the only one with the authority to interpret the Bible, which is an obvious red flag. But even lacking this, the Bible tells us that it is sufficient for all matters of faith and practice. There is nothing a Christian needs to know that the Bible cannot tell him (2 Timothy 3:15-17). Thus, if you want to know what Christianity is, you have everything you need, just read your Bible. It soon becomes very obvious who's Christian and who's not.