If you're going to make an argument against Christianity, that argument must do one of two things. The first is to deny it by showing a lack of necessary evidence. I'm not talking in the sense of "absence of evidence is evidence of absence" (though many atheists take such a silly approach). When I say necessary evidence, I mean evidence that must be present if Christianity is true. This is analogous to a shooting. If you got shot in the stomach, there would be a bullet hole. It is therefore sound logic to say "you have no wounds, therefore you were not shot". It is not, by contrast, sound logic to say "there are no references to the historical Jesus outside the Bible, therefore He did not exist", because 1. There are, 2. The Bible does count, in spite of your bias against it, and 3. extrabiblical references are not necessary evidence, they're just helpful.
The alternative is to provide contrary evidence which, when presented, leads, or at least contributes to, the conclusion "therefore, Christianity is not true". For example, what the Pharisees should have done, rather than scourging and killing the disciples for claiming Jesus rose, would have been to collect His body and parade it around town, showing indisputably that He was still dead, and had not risen. That would have been positive evidence against Christianity.
It's amazing, then, that so many arguments against Christianity do not depend on denying necessary evidence, nor on presenting contrary evidence, but on showing that, at least in part, the Bible is true. The Bible says X is true, the unbeliever says X is true, yet the unbeliever, beyond all imagination, says X disproves the Bible. Let's look at 3 examples.
The decline of Christianity
The ad populum fallacy is a well known fallacy among anyone who has ever had a responsible parent. We're all familiar with the phrase "if everyone jumped off a bridge, would you do it too?" But apparently, when it comes to faith, logic goes out the window, as many atheists argue the decline of Christianity (in the West) is evidence of its falsehood. Problem number two with this argument is that scripture not only predicts the eventual decline of Christianity, but even that when Christ returns, He may not find faith on the Earth (Luke 18:8). The foretold "Great Apostasy", a.k.a. "the Falling Away", must take place before the end. Therefore, any Christian who has ever paid any attention to the New Testament, especially 1 Thessalonians, has expected a decline, even a sudden collapse, in Christianity. How, then, is it even remotely realistic to say "there is a decline in Christianity, just like the Bible says, therefore the Bible is not true"?
The "problem" of evil
The "problem" of evil is probably the strongest and most effective argument against Christianity because it both pulls at people's hearts while simultaneously appearing logical. The problem of evil attempts to show that God and evil cannot logically coexist, and so since evil does exist, God cannot. Some unbelievers like to add to this by being more specific, such as showing that even prayer doesn't stop evil, and even children die of cancer.
But this is based on a severe misunderstanding of God. Unbelievers who make this argument typically see God as this glorified cuddle merchant who can do even the logically impossible. Of course, the first problem with this understanding is that such a being would be logically immune to such criticisms. Any being capable of doing the logically impossible could be pure evil and perfectly Holy at all times. Thus, He could directly commit every possible act of evil at every opportunity and still be 100% good.
But this is not the God described in scripture, as while God has no external limits, He is nevertheless limited by His own nature. This is why scripture is able to say things like "In hope of eternal life, which God, that cannot lie, promised before the world began;" (Titus 1:2) or "If we believe not, yet he abideth faithful: he cannot deny himself." (2 Timothy 2:13). The inability to deny Himself prevents Him from doing illogical things, such as creating free beings without the ability to do evil. Yet, scripture tells us two things very clearly: 1. God created us free, and 2. We often abuse that freedom.
Indeed, it is that very abuse of freedom that caused the Earth to fall into a state of disrepair in the first place. When Adam sinned, God cursed the word with death and decay, and "...death passed upon all men, for that all have sinned:" (Romans 5:12b). Furthermore, scripture makes it clear that although we differ relationally with God than the impenitent, we are not immune from this while on Earth. Instead, it tells us that in this life, we will have trouble, and that often this is even because of our faith.
Ultimately, the "problem" of evil has a number of both simple and nuanced answers, but the long and short of it is the world as we see it is exactly as we would expect to see it if the Bible is true. Thus, once again, unbelievers are attempting to criticise the Bible by granting its truth.
Evidence of paganism in ancient Israel
If, in some future time, archaeologists were to examine my house, they may well find my Qur'an. How sensible would it be for them to conclude there were never any Christians living here? Obviously, this reasoning is absurd. Yet, whenever evidence is discovered in Israel of some ancient pagan religions, this is said to show Judaism originated much later than it actually did. But positive evidence that the ancient Israelites worshipped other gods does not make a dent in the accuracy of the Bible, first of all because multiculturalism has always been a thing, but second of all because the Old Testament shows a never ending cycle of apostasy and faithfulness. During Elijah's time, things got so bad, a mere 7,000 Jews had stayed faithful to God (1 Kings 19:18; Romans 11:4). Thus, it is absolutely staggering that anyone would say "hey, we found evidence of paganism in ancient Israel, just like the Bible says, so obviously the Bible isn't true".
These three examples are far from alone. All too often, unbelievers seem to think showing evidence that the Bible is true actually shows it is not. This abysmal logic could actually be considered soft evidence that Christianity is true. After all, if it wasn't, would unbelievers be making such desperate attempts to show otherwise?