The devil was sick …

The devil was sick, the devil a monk would be;
The devil was well, the devil a monk was he.

People who make pious resolutions in time of illness or peril forget all about them when they are healthy and safe again. John Owen wrote this epigram:

God and the Doctor are alike adore
But only when in danger, not before;
The danger o’er, both are alike requited,
God is forgotten, the Doctor slighted.

The phrase ‘the devil a monk was he’ is a play on words; it does not mean that the devil became a monk, but that he did not. We find it in such other contexts as: ‘I asked everybody to attend the meeting, but devil a one turned up.’ Nobody came.