"main tenet" is correct and usable in written English.
You can use it to refer to a fundamental principle, such as, "The main tenet of this philosophy is that our actions determine our fate."
The main tenet of the Dogs Trust is horribly reprehensible.
Its main tenet is that the periphery of the international economy is being economically exploited (drained) by the centre.
Not all Egyptians like the group or want to see it in charge, but few argue with its main tenet: "Islam is the solution".
These bannings may illustrate a main tenet of Mr. Liu's writings: that China today is "post-totalitarian," but still a dictatorship.
But in defiance of a main tenet of mainstream economics, that people make rational decisions in their best interest, hardly anyone returned the checks.
To be sure, his main tenet — that supply-side economics with pervasive tax cuts would generate revenue and assure growth — is dubious.
Iran's flag contains a five-pronged shape, believed to represent the main tenets of Islam.
Contrary to appearances, the law's main tenets are unlikely to be abandoned completely.
The main tenets of the Carson credo were these: (1) Learn to laugh at yourself.
But the individual mandate is one of the reform's main tenets.
Mr. Rumsfeld plans to preview the main tenets of the Pentagon's thinking to Congress next week.
