At least one among Islam and Christianity has to be wrong. Islam says that Jesus was just a human Prophet (and a slave of Allah) and NOT Son of God and he escaped crucification, and that Muhammad was the best and last of God's prophets. Christianity says that Jesus was not just a human Prophet but SON OF GOD, and that Muhammad was a nobody, i.e. a hoax and that Jesus was crucified. Now at least one of these two beliefs has to be wrong, and both of them can also be wrong. As well-known scholar Koenraad Elst, a born Christian (who was so intensely religious as a child that he even considered becoming a missionary!) says, actually, they are.
Now, so many people have been killed by followers of Islam and Christianity genuinely believing that are doing the work of God. The Quran is full of verses giving terrible warnings of Hell to unbelievers, and urging believers to fight the unbelievers and convert or kill them (like 9:5, 2:193, 8:39, 8:12, 8:65, 9:17, 9:28, 9:29, 9:39, 9:110, 9:123, 5:33, etc etc ). Now misguided followers have killed millions in the name of spreading faith in God. Christians and Muslims too have clashed among themselves and killed millions of people. A true God would have stopped at least one of them from killing innocents in his name. For example, if Christianity is true and Islam false, then Islam killing millions of people in the world, including at least 280 million in India, in the name of God would not have been allowed by God. God would have made his misguided followers realize that he does not want killings in his name. In the same way, if Islam was right and Christianity was wrong (in asserting that Jesus was SON OF GOD) a true God would not have allowed misguided Christians to kill millions of people in his name (like the Christians did in South America, killing some 100 million people). He would have made them know that Jesus was NOT his son but just a human Prophet and that they needn't kill 100 million innocent people in his name. But since God did not do anything like this, it is clear that God, if he exists, does not interfere in human affairs.
As said earlier, the Creator focusing his attention on tiny humans on earth is like we humans focusing our attention on tiny invisible viruses living on a drop of water somewhere down the Pacific Ocean. It is these tiny viruses who kill each other in the name of God to satisfy their own wants, God doesn't force people to believe in one of those tiny viruses and his claims of being God's messenger.