Erasmus, Adagia, 1.1.15:
“Tyndarus, one of the captives in the play of that name by Plautus, was caught red-handed in the middle of his scheming. Having no device by which he could escape, he said
Now I am utterly destroyed. Now I stand between the altar and the knife, and I don’t know what to do.
Apuleius, in the eleventh book of his Golden Ass, writes:
At a time when the hardness of poverty was interfering with my life, I was – as the ancient proverb has it – being put to torture between the altar and the knife .
Apuleius, however, explains the saying allegorically as referring to the priesthood to which he was about to be initiated, and the poverty which was harder than a rock, on account which no resources were at hand. It is clear that this has been taken from the earliest ceremonies of striking up a treaty, in which the Fetial would strike a pig while pronouncing these words: ‘whoever breaks this treaty first, let Jupiter smite him just as I smite this pig with this rock.’ But though the proverb has flowed this way and that, it is clear enough that it was usually applied to those who, in their perplexity, are driven to the most extreme danger.”
Inter sacrum et saxum.xv
Tyndarus apud Plautum, alter e captivis, cum jam proditis dolis esset deprehensus nec haberet, quanam arte possit elabi,
Nunc ego, inquit, omnino occidi.
Nunc ego inter saxum sacrumque sto, nec quid faciam scio.
Apuleius Asini sui libro undecimo:
Plurimum ergo duritia paupertatis intercedente, quod ait vetus proverbium, inter sacrum et saxum positus cruciabar.
Explicat autem Apuleius allegoriam adagii videlicet alludens ad sacerdotium, cui erat initiandus, et paupertatem saxo duriorem, per quo non suppetebant sumptus. Sumptum apparet ex priscis foederis feriendi ceremoniis, in quibus fecialis porcum saxo feriebat haec interim pronuntians : Qui prior populus foedus rumpet, Jupiter eum feriat, quemadmodum ego porcum hoc lapide ferio. Sed undecumque fluxit adagium, satis liquet dici solitum in eos, qui perplexi ad extremum periculum rediguntur.
It should be noted that the word you’ve translated as ‘knife’, ‘saxum’, actually means ‘rock’ (as is clear from the description of the ritual). This could be explained as making the text easier to understand, but an idiom like this is hard to understand in any translation without a note on its meaning.