The speaker admits to going on a "great rant" in defense of "what was truth," and the past tense in the title and the poem does a lot of work. Truth here isn't a living thing being argued over. It's something that used to hold, and the speaker is now defending its memory.
Then the poem turns on itself. The speaker calls the whole performance "an ingenuous sort of entertainment / without much cost." The word "ingenuous" suggests something naive or openly sincere, and "without much cost" makes the rant sound cheap, almost harmless. Judging by the tone, the speaker sees how little such outrage accomplishes, and maybe how easy it is to put on a show of passion.
The mood is wry and a little tired. There's self-mockery in it, but also a quiet sadness: the speaker still cares about truth enough to rant, yet knows the rant is a small, inexpensive thing.