Truth in "The Second Nun's Tale" refers to Christianity. The tale presents Christianity as necessarily more true than any other religion because of its belief in an eternal God who grants eternal life. This conception of truth draws upon the Greek philosopher Plato's idea of truth as something that is always true and unchanging – hence, an eternal God, and a life in him, is more true than any other kind of god or life. A late classical or medieval person familiar with this concept would find completely reasonable Tiburtius and Valerian's declaration of Christianity as the truest thing they've ever heard, even though to us their certainty might seem mysterious.
Another characteristic of truth in "The Second Nun's Tale" is that it is exclusive – that is, not everyone can easily obtain it. Only those who give up their former attachments (represented here by pagan idols) and purge themselves of sin are able to really know truth, and these steps may be possible to them only through God's grace. Those like Almachius, who have no desire to take these required steps, will never be able to recognize truth.
"The Second Nun's Tale" portrays truth as accessible only through God's grace.
"The Second Nun's Tale" portrays truth as something that can be reached through the actions of the truth-seeker.
Cecilia's failure to die when Almachius orders it symbolizes Christianity's evacuation of all truth from Almachius and his statements.