I think the most compelling argument for me is in the NT rather than the Old. Why? Because I think its quite easy to prove that the Old Testament does not mention or speak about hell or everlasting punishment (many Christian scholars admit this today, even staunch ECTers). So the battle is waged in the New Testament definitions and man are there some very literal and good verses for “aionios”. A typical argument for “aionios” always meaning eternal is Matthew 25:46, where eternal punishment is paralleled with eternal life. For Augustine this was the ace in the hole for ECT, and ever since it has been what mostly silenced the idea of UR. Here’s the cool bit, there are a few verses that parallel two uses of the word “aionios”, and in each passage one “aionios” has to mean “of the ages”. Here let me lay it out so you can understand what I’m saying:
Matthew 25:46
And these will go away into eternal punishment, but the righteous into eternal life.
καὶ ἀπελεύσονται οὗτοι εἰς κόλασιν αἰώνιον (kolasis aionios), οἱ δὲ δίκαιοι εἰς ζωὴν αἰώνιον (zoe aionios).
Titus 1:2
in hope of eternal life, which God, who never lies, promised before the ages began
ἐπ’ ἐλπίδι ζωῆς αἰωνίου (zoe aioniou), ἣν ἐπηγγείλατο ὁ ἀψευδὴς Θεὸς πρὸ χρόνων αἰωνίων (pro chronon aionion)
Here we have eternal(aionios) life in the same sentence where a second use of aionios cannot mean eternal. Pro chronon aionion, this literally means “before the times of the ages”, and the translators cannot help but conclude that this is the only sensible translation, only in a few is it translated “before times eternal” but again this is an impossible meaning, there are not before times eternal. And even if someone wanted to say, "Oh he’s talking about eternity in the future after the ages are done, the context disproves it. In the next sentence it says that this eternal life “was at the proper time manifested”, meaning what was promised before the times of the ages (or ‘before times eternal’ if people want to be consistent) was manifested in Jesus Christ. So here we have a parallel just like Matthew 25:46, only this time its clear that aionios does not mean eternal even when set beside aionios life.