I have often used the analogy that if a modern man claimed to be Jesus and gained a significant number of followers, those who followed him would have no legitimate claim to be Christians because the Jesus they follow is not the Jesus of Christianity. Therefore, it makes no sense to classify various religions, especially those that sprung up in the 18th century, as a "Christian" denomination, since any Jesus that isn't the Biblical Jesus, whether it be a real person or an imagined caricature, has no legitimate connection with the Christian faith.
I was rather surprised, though perhaps I should not have been, to find out that actually, many people throughout the ages, even to this very day, have made this very claim. My analogy is no longer hypothetical, but is in fact backed by a real world phenomenon.
But whether real or imagined, I am neither the first to use it, nor am I the most authoritative (to be clear, I am not claiming any authority over the Church). Rather, the Lord Himself used this concept through Paul the Apostle in 2 Corinthians 11:1-4. In this passage, Paul expresses his disappointment at how malleable the faith of the Corinthians is. They had become so open minded that their brains had fallen out. They had previously accepted the Gospel, but now heresies were creeping in bit by bit.
Another Jesus, a different spirit, and a different gospel = a different religion. Keep in mind this has been happening since the first century, so not even the age of a denomination is enough to give it legitimacy. So, what does give a denomination legitimacy? The same way every religion defines itself: through its authoritative texts! God didn't leave man fumbling around in the dark. There are 66 books (depending how you separate them) which have been inspired by God, and within these books is a full description of the Christian faith. There are "lesser" and "greater" doctrines within it. There are, however, three essential elements: Jesus, the spirit, and the Gospel. Any religion which removes these elements is no more Christianity than Islam, a religion which, while it claims to be from the God of the Bible, makes no attempt to call itself a Christian denomination.
It is the official position of this ministry that there are true Christians in every denomination, and that even some denominations satisfy all the essential elements required to be called Christianity. However, there are no authoritative denominations, and where a denomination deviates from the scriptures, it ceases to be Christian.