Novice said:
Timothy, there is One Truth. That Truth is God.
Insert that equivalence into your sentence, and try the result on for size:
"There must there must be a definable [God], or else God is not the God he claims to be."
Where did GOD ever claim (or offer) to be "definable", and what man who ever loved Him would ever dream of trying? The ancient Hebrews dared not even speak His name, lest that be seen to somehow limit or define Him. Yet this is the outgrowth of the Bibliolatry of our day: we have confused the word of God (which can be studied and analyzed) with the WORD of God, who "before Abraham was, I AM".
As C.S. Lewis once wrote: "He is not a tame lion."
"Knowledge puffs up, but love builds up. The man who thinks he knows something does not yet know as he ought to know. But the man who loves God is known by God."
I think what needs to be distinguished is the revelation of God to man, and the definition or encapsulation of God. Truly God is all powerful and ever merciful, but those two descriptors do not exhaustively describe God. But they are nonetheless true as revealed qualities of God- or rather, revealed descriptors.
It is true that the bible has been misused to such an extent that I have heard people teach that God is limited to what is written in the bible. Still, scripture is the measure of truth (canon), so when the Psalmist says God is good, and His mercy endures forever, this is absolutely true, despite our experiences which might lead us to believe elsewise.
Even your defense of God as unlimitable, and in most senses unknowable, is rooted in an absolute sense of truth that you have, which is that God is infinitely beyond our finite capacity to understand. This is an axiomatic statement which declares an absolute truth.
It is difficult to make positive absolutist statements about God, so our best efforts are typically apophatic (negative)
For example: God is not a liar, God has neither shift nor shadow. God is not a creation.
But again we find ourself grappling with concepts of God. But the OP spoke also about truth being "objective and knowable," rather than "a subjective and personal idea." Now we must begin with an exploration of whether truth , truths, or Truth are objective or subjective only.