Sometimes, the word, absolutism, is used as if it were the opposite of relativism or post-modernism.   It is not.   Absolutism has two meanings.

• Absolutism: adherence to a set of rules, laws, ethical standards, moral precepts, etc.
• Absolutism: belief in a strong central government with absolute power.

Yikes!   Absolutism does not work very well as an antonym for relativism or post-modernism.

The word, fundamentalism, doesn't work very well as an antonym for post-modernism and relativism either.   Two separate groups who created two separate fundamentalisms in the early 1900s originally created the word.   Each of them had its own set of claimed "fundamental truths."   As time went by, journalists, for whatever reason, twisted these words to describe both terrorists and any Christian who believed the Bible was the Word of God, neither of which was an legitimate use of the word, fundamentalist.   Perhaps, in the minds of Secularist journalists, Christians are evil and terrorists are evil.   Who knows?

In today's world, the word, fundamentalist has come to simply mean "a religion you don't like."   If you don't like a certain religion, call them fundamentalist.   It's an epithet.

We could certainly use a new name that is the opposite of post-modernism and relativism, but those disciplines would then muddy the meaning of the new name, because that is what they do.  

Dogmatism won't do.   The most dogmatic people on the planet are the relativists and the post modernists.   Secularists, Evolutionists, and Naturalists are certainly dogmatic.   Liberals are only open to liberal ideas and are also very dogmatic.

God has revealed some things.   Those things, we know.   There are many things that we don't know and many things that we only partially know.   What is the word that expresses that concept?  God is continuing to reveal Himself and His ways to everyone who desires to be obedient to Him in everything.