I prefer to think of it like a jigsaw puzzle. It might be that different faiths have a different number of pieces to work with. Obviously I think Christianity has the most pieces (not all of them by any means), which would mean Jews and Muslims are missing a couple that Christians have, while Hindus and Sikhs have maybe even less.
As long as we're all building the same picture, then there's no problem. We're all looking for the same truth and each have some amount of it.
Trouble is, I think Islam might be building a different picture. And I'll sum up a few reasons here.
Before I get going, I want to make it clear that I don't think this means all Muslims are stupid, evil, or bad in any way. And I'm not touching the subject of whether or not Islam is a religion of peace or war. I'm just putting out there some issues with the faith.
Maybe Muslims have good explanations to put me straight, but so far I haven't heard them.
Contradicting The BibleYeah. This first problem comes from being a Christian so it's meaningless to anyone who isn't one.
Basically, the centre point of Christianity is the crucifixion and resurrection of Jesus: the man who was God. Islam teaches that neither of these things happened and that Jesus was a prophet, not a part of a Godhead.
So if Christianity is true (which I'm convinced it is), then Islam simply can't be. The contradiction is too big a deal. If the contradiction had been something like Moses spent 3 years in the desert instead of 40, then that's not a problem. Who cares who's right? The core message of the story still stands. But denying Jesus divinity and his resurrection is impossible to compromise on.
As I said, this objection only counts from a Christian perspective. To an outsider of both worldviews, it's probably meaningless.
Contradicting HistoryThis next point links from the last. Jesus crucifixion is a solid historical fact. In secular society, the resurrection is still under debate despite there not being a reasonable alternative explanation, but the execution itself definitely, 100%, without a doubt happened.
Islam denies it. It's plain wrong. The Quran says that everyone was deceived and an imposter was killed in Jesus' place. Never mind the public trials and beatings where Jesus was clearly seen by everyone. My biggest issue with this explanation is that it makes God out to be a liar. He had a prophet sent to Earth to bring a message of salvation and self sacrifice who ducked out when the going got tough and let someone else suffer and die in his place. God then led everyone to believe Jesus had been killed.
Aside from this solid contradiction, the question of whether Muhammad even existed is still up for debate. If he didn't exist, then that's another mark against Islam.
Supposed To Be God GivenUnlike the Bible, the Quran is meant to be written by God through Muhammad. This simple fact means that it can't contain any mistakes. If there is one mistake then it's false - because God doesn't make mistakes. The Bible doesn't have this problem. It's written by men who are divinely inspired. They aren't writing as God dictates to them, they're putting it into their own words. Mistakes can creep in.
Seeing as the Quran contradicts historical knowledge, it must be wrong.
Contradicts ItselfThere are a few of these but I'll pick a good one.
"Believers, Jews, Sabaeans and Christians - whoever believes in God and the Last Day and does what is right - shall have nothing to fear or regret." Sura 5:69These two passages are three verses apart! You'd think someone would notice. The first says Christians will go to Heaven. The second says that Christians will go to Hell. Could not possibly be more of a contradiction!
"Unbelievers are those that say: "God is the Messiah, the son of Mary." For the Messiah himself said: "Children of Israel, serve God, my Lord and your Lord." He that worships other gods besides God, God will deny him Paradise, and the fire shall be his home. None shall help the evil-doers." Sura 5:72
The Islamic DilemmaProbably the biggest problem for Islam is this.
In a number of places, it confirms that the Gospel and Torah were given to Christians and Jews by God. They should use their books to learn from because they were divinely inspired. Even Muhammad himself was guided by the Bible.
"But if you (Oh Muhammad) are in doubt as to what we have revealed to you, ask those who read the book before you; certainly the truth has come to you from the Lord, therefore you should not be of the disputers." Sura 10:94
Islam has two options: Either the Bible is divinely inspired or it isn't.
If the Bible is divinely inspired then Islam can't be true because it contradicts it.
If the Bible is not divinely inspired or was corrupted then Islam can't be true because it wouldn't guide people to read it.
The Quran tells Christians that they should follow the Gospel and the Muslims should use the Gospel to confirm their own scripture. But seeing as the Gospel and Quran are different in a number of irreconcilable fundamental ways, they can't both be true. The obvious choice for the wrong one is the Quran, because it tells you to study the Gospel!