Atheists are not known for their consistency. Aside from their arguments never really being logically sound when taken alone, many of them contradict each other. Take, for example, the two in the header image. On the one hand, atheists use the existence of numerous denominations to say that since they can't all be right, they must all be wrong. On the other hand, some of those denominations are morally grotesque in some way or another, so surely Christianity as a whole is evil, right?
While the existence of multiple denominations is certainly a problem for Christianity, it is not a problem in the logical sense. While all denominations cannot be true, each denomination has some degree of truth. Many denominations agree on "the basics", as in we all have the same Gospel, and disagree only on minor things, such as infant baptism, or Calvinism. It is absolutely permissible for different Christians to have different opinions on minor issues (e.g. Romans 14:1). Even some of the bigger issues are not so big as to say that because the two contradict, therefore Christianity is not true.
Not that this would be logical anyway. After all, atheism contradicts Christianity, yet one cannot say "we can't both be right, therefore neither of us are". There is not a single issue on this earth on which there is universal agreement. If two statements contradict, it just means one of them must be false, not both.
But while atheists argue that Christian denominations are all so radically different that they can't all be true, they also argue that they are all so indistinguishable that any evil action performed by anyone even remotely linked to a denomination that merely calls itself Christian must necessarily be blamed on Christianity.
There are several major flaws in this argument. First, while not all denominations are totally incompatible with each other, several of them are so incompatible with the Bible that it is impossible to consider them Christian. In 2 Corinthians 11:4, God, through Paul, talks about people who preach another Jesus, a different spirit, and a different Gospel. In Galatians 1:8, Paul also says that neither he, nor the other Apostles, nor even an angel can change the Gospel, and basically says if anyone tries, may they go to Hell. So, even if a denomination calls itself Christian, that doesn't mean it is, and thus blaming Christianity for its actions is as absurd as blaming Christianity for Islam.
Second, even if a denomination is truly Christian, a person's actions are not necessarily reflective of their faith. This is especially the case with Christianity, which states that we are all sinners, and therefore it is inevitable that even the "best" of Christians will do some very unChristian things. I, for example, openly confess that the... "colorful" language I can often be heard using is decidedly unfitting for a Christian to use. The fact that I swear a lot is not reflective of my faith, it's reflective of my frailty as a human being.
Third, atheists conveniently remember this when it comes to evils committed by atheists. Over the past century alone, millions of lives have been lost to atheistic regimes. Atheists, being allowed by their philosophy to make up their own rules, can be anything from the nicest people you'll ever meet to flat out mass murderers. Yet no atheist will blame atheism when atheists do the same evil things as religious people, or worse. Convenient how that works, isn't it?
Both of these arguments are ultimately contradictory. You cannot say all Christian denominations are so different as to be mutually exclusive, yet so similar as to deserve to be held collectively responsible for each other's crimes. Not only are these arguments contradictory, but even if an atheist wanted to be consistent and pick only one of them, both arguments fail because they simply aren't logical. Neither of them necessarily lead to the conclusion "therefore, Christianity is false", and in fact both of them reveal that the one making the argument does not even understand what Christianity is.
Christianity, as the name suggests, is all about Jesus Christ. Not just any Jesus, for the world has made up many of those, and as shown above, the Bible even mentions this fact. The Jesus of Christianity is a very specific Jesus. That is, the historical, Biblical Jesus, and the Gospel He preached.
The Gospel is relatively simple. It literally means "good news", and as is often the nature of good news, it starts with bad news that needs to be rectified. The bad news is that human beings are all sinners, which means we are in rebellion to the God who created us. We disobey His commands, and this disobedience merits death, both physical and spiritual.
But the good news is that God takes no pleasure in death, preferring the wicked repent so that He may grant them eternal life. 2,000 years ago, He sent Jesus, His Son, to be born of a virgin, life a perfect human life, and die for our sins on the cross. Jesus then rose from the grave so that all who believe in His name can be completely forgiven, and inherit eternal life in His Kingdom when this life is over. The Gospel can be summed up in two sentences: Jesus stood before God for your crimes so that you, through faith, can stand before God as if you were Him.