Of truths we say correctly “Once true, ever after true”. All truth is in that sense inconcussible and exempt from vicissitude. It achieves what Mr. Whitehead calls “objective immortality”, even if the truth be about temporal events that have occurred, and about their temporality. If it be true that X occurred, that truth cannot afterwards be altered.... Truths, therefore, that describe present and past (including truths that describe their temporal relationships) are, because true, timelessly so....

Similar remarks should be made about the eternity of concepts; for concepts themselves are timeless or unchangeable. This applies even to the concept of time or to the concept of change. If by reason of much philosophy we come to alter our conception of time or of change, we may or may not be wise, but the meaning of the statement is that we come to use different concepts to describe these realities or what we suppose to be such. The concepts do not change. We choose different concepts to express what we mean; and the facts may justify us.   —John Laird

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Eternal:
Middle English, from Middle French, from Late Latin aeternalis, from Latin aeternus eternal, from aevum age, eternity

a : having infinite duration; everlasting
b : characterized by abiding fellowship with God
c : continued without interruption; perpetual

Truth:
Middle English trewthe, from Old English treowth fidelity; akin to Old English treowe faithful
True: Middle English trewe, from Old English treowe faithful; akin to Old High German gitriuwi faithful, Old Irish derb sure, and probably to Sanskrit daruna hard, daru wood

a : fidelity, constancy
b : sincerity in action, character, and utterance
c : in accordance with the actual state of affairs; in accord with fact
d : conformable to essential reality
e : the body of real things, events, and facts
f: transcendent fundamental or spiritual reality
g : a judgment, proposition, or idea that is factual or accepted as proper
h : fidelity to a standard
i : logically necessary