A common objection by unbelievers is that there are too many Christian denominations for them all to be true. To answer these people, I simply say that they are absolutely correct. Not all Christian denominations are true. In fact, a significantly high number of them are not.
The majority of denominations share such trivial differences that they can barely be considered different denominations. Members of each denomination hold to all the same doctrines, with only a few minor disagreements that never really affect their faith in any measurable way. If you meet someone who primarily identifies as Christian, the chances are they will differ as slightly from members of other denominations as they do from members within that denomination. They will even (depending on how long they spend studying Christianity) understand most of the "inside jokes" they make.
But what of the big disputes? What about when a fundamental issue of faith is the cause of the disagreement? In no uncertain terms, Paul says that such disputes are not fitting for Christianity. Specifically, in 2 Corinthians 11:3-4, Paul compares such disputes to Satan's deception of Eve in the Garden of Eden. He then gives three essential elements to watch out for: Another Jesus, a different spirit or a different Gospel.
There are hundreds of different religions that claim to be Christian that are preaching a different Jesus. All throughout history, there have even been people who have claimed to be Jesus, or have had that claim made about them. For example, in November 2011, Oscar Ramiro Ortega-Hernandez attempted to kill Barack Obama by firing on the White House. He later claimed to be Jesus, and Obama was supposedly the anti-Christ, whom Jesus was sent to kill. To my knowledge, Oscar Ramiro Ortega-Hernandez has no visible followers. Other false Christs, however, have succeeded in gaining some following.
A more successful example would be the World Mission Society Church of God, a cult which believes Ahn Sahng-hong to have been Jesus (Ahn Sahng-hong himself never made this claim, but instead claimed there would be a second coming before the end of 1966). When I last checked, the World Mission Society Church of God had 1.7 million members, and may have significantly more now. However, despite its claims, it is not a Christian denomination, but a heretical group that can even be legitimately described as a cult. It is safe to say it is not Christian because it is based on another Jesus.
The other Jesus that a denomination preaches does not have to be physical to qualify that denomination as heretical. An imaginary figure can be called Jesus and not be the same as the Jesus of history. This automatically erases a wide variety of false denominations.
Another element of a denomination that can invalidate it as Christian is the spirit. Galatians 5:22 gives us a helpful guide for how to recognise when a denomination has the correct spirit by describing its fruits: love, joy, peace, longsuffering, kindness, goodness, faithfulness, gentleness, self-control. A denomination that lacks these fruits, by definition, cannot be Christian. It makes sense, then, to discount the infamous Westboro Baptist Church, a hateful, violent, unkind group of radicals with such an extreme lack of self control that they protest at a funeral!
The final element, which is arguably the most important element as it encompasses the other two, is the Gospel. Bad Gospel, bad denomination. If your "gospel" involves us becoming gods, like Mormonism, your religion is not Christian. If your "gospel" leaves Jesus dead forever, like that of the Watchtower Society, your religion is not Christian. If your "gospel" requires you to work for salvation, and condemns you for failing to do so, such as that of the Catholic Church, your religion is not Christian.*
All of the religions discussed above have claimed to be Christianity, and even the only true Christianity, yet a simple Bible search (which has been made far easier in our modern day thanks to a variety of apps, websites and search engines) can refute that claim. To my unbelieving friends who say not all denominations can be correct, I say this: You're right. And if (hopefully when) you become a Christian, you need not join one. If you can find a Bible-based church in your area, do so, but you need not assume the denomination they claim ought to be your own. The Bible is the ultimate authority in Christianity. No denomination can claim to be higher.
*Please note: According to the Bible Brain Statement of Faith, "There are true Christians in every denomination, but there are no true Christian denominations." Therefore, although this article has singled out several denominations, and utterly rejected them as even potentially Christian, this does not mean that someone who identifies with these denominations is necessarily non-Christian.