All gods are aspects of the same God
"All gods are aspects of the same god" is a proposition that some more liberal theists use to explain away the Argument from inconsistent revelations. It is commonly stated by Hindu and New Age believers, as well as by apologists who are no longer strongly associated with a particular religious tradition.
One can see that Islam, Hinduism, and Christianity often use similar arguments and psychological tricks, but that none is particularly more convincing than the others. Most believers are either unaware of this problem, or ignore it, or argue that some particular argument works better for them than for other religions. Alternatively, an atheist concludes that they are probably all wrong and disbelieves in all gods until one religion comes forward with with more convincing evidence.
However, some liberal theists, particularly those with misguided multiculturalist intentions, often try to take the reverse tactic and believe that every religion is true. Often these same people will claim that criticizing any particular religious tradition (or stating that any particular one is superior) is a form of religious intolerance or bigotry that denigrates others' beliefs.
These people are then in a bind, where they can't accept every religious statement that comes along without being extremely gullible and/or seeming to pick favorites. On the other hand, they can't reject these statements without feeling bad about "denigrating" other people's beliefs. They are consigned to a sort of murky assent, where they claim that somehow, perhaps "mysteriously", all the things that different people believe are true, despite how bizarre many of these claims are.
The religions themselves are incompatible and intolerant
First off, it should be noted that almost any pair of traditional religions you care to name are on some level incompatible with each other. Hindus may be willing to accept a lot of different gods, for example, but for that very reason Hinduism is incompatible with every major Abrahamic religion. This is true not only in the sense that they tell different stories, but also because their moral commandments and practices are in direct opposition. When Yahweh demands that only he may be worshiped and repeatedly commands his followers to drive out and kill worshipers of other gods, destroying their temples and religious artifacts in the process, that's not a trivial inconsistency.
The truth is, religious intolerance of one form or another is part and parcel of most religions, and to deny this is to be woefully ignorant of basic history. If all these various gods are aspects of one God, He must be crazy, cruel, or both, because His extremely inconsistent revelations and denigration of all His other religions have led to countless wars and lives lost over thousands of years.
You don't speak for all theists
Despite the feel-good message of tolerance and respect in this message, it is also very presumptuous. Walking into the average church, mosque, or (non-polytheistic) temple, and saying something like "All gods are the same God" would probably not get you a whole lot of supporters. You can't just say "you believe in a God that's the same as every other god" to most religious people without being rather rude, because it's intrinsically arrogant to tell other people what they believe.
Anyone who thinks that most religious people really would agree with a line like that needs to get some broader experience, because that's not how the world works right now.
Not all gods are the same
Even though people do believe in some sort of god, some people believe animals and/or idols are god/s. This goes against the definition of what most religions define god to be.
What about Polytheism?
Polytheism, or the belief of multiple gods, contradict what most religions when they say that there's only one god.