A straw man argument is when an opponent of a position misrepresents that position, or an argument in its favor, and proceeds to attack their twisted version rather than the actual position. For example, when a pro-gunner argues that owning a gun significantly increases your chances of self defence, an anti-gunner will often respond by pointing to an example where a gun could not, or did not, save a person's life. The anti-gunner has constructed a straw man by suggesting the pro-gunner believes guns make one invincible, when in reality, pro-gunners just know that a person who owns a gun, while not invincible, has a significantly higher chance of defending against a violent crime.
Christianity faces this kind of argument all the time, but for Christianity, it goes beyond opponents simply constructing a false picture of Christianity and tearing it down. Instead, such false pictures have already been constructed, and have persisted, in some cases, for almost as long as Christianity itself. This is called a "denomination". That is, a religion which claims the Bible as its foundation, while more often than not running contrary to it.
Consider as an example the following quote from Adolf Hitler: "The Christian-Jewish pestilence is surely approaching its end now. It is simply dreadful, that a religion has even been possible, that literally eats its God in Holy communion." Hitler firmly believed that his argument against communion was a valid argument against Christianity (and, for some unknown reason, Judaism). However, Hitler's argument, which many atheists still use today, only works against one very specific denomination (and its offshoots): Roman Catholicism. Catholicism and Christianity are worlds apart on a number of issues. If everyone in the world universally agreed that Catholicism is false, that would be a positive thing for Christianity, not a negative. You cannot refute, or even slightly damage Christianity, by refuting Catholicism.
Unfortunately, when called out on a straw man argument, unbelievers will often double down on it. You point out that Catholicism is not Christianity, the unbeliever will say that it must be, because Catholics say it is. There has never been a more ridiculous standard of logic, but a Christian can afford to play ball here. For a moment, it is possible to pretend that the Bible doesn't exist, so nothing in Catholicism that is anti-Christian matters. That still doesn't change the fact that the aforementioned Christian is not Catholic. Bible Brain, for example, is very obviously not a Catholic ministry. It never has been, and as long as it is within my power to prevent it, it never will be. If you want to debate Catholicism, go find a Catholic. I'll even give you a few pointers if you like. But when you say something to me like "your religion is disgusting because it teaches that you must eat God", and I tell you that that's a ridiculous misrepresentation of my views, it doesn't matter whether or not you erroneously believe Catholicism is a truly Christian denomination, you are still misrepresenting my views, which are similar to the views held by Christians since before the New Testament was even completed. Defending your right to misrepresent my views is dishonest.
The defence of a straw man argument is indicative of a lack of valid arguments. If you have a good argument, you don't spend significant amounts of time defending a bad one. No matter which way you cut the pie, if an unbeliever's arguments only work on one denomination, or even on more than half of them, that's still not significant as a refutation of the Bible. You cannot refute Christianity by refuting those who have made up their own version. In any debate, an accurate representation of one's opponent is essential. If the only way you can refute someone's worldview is by misrepresenting it, it's time to stop fighting and admit defeat.