'Resting' Catholics: now that is a phrase I haven't heard before, though I have heard that actors who are not working, i.e. out of work, are sometimes euphemistically described as 'resting'.
Presumably, a 'resting' Catholic is one who temporarily, (or over a long time),is not practising his faith.
Bishop Conry has reminded Catholic mothers, of the need to pray for Catholics who have ceased to practice their faith. At the launch of the annual 'Come home for Christmas' campaign, Bishop Conry suggests that we might pray for the powerful intercession of St. Monica, the mother of St Augustine of Hippo. St Monica's incessant prayers for her, then wayward, son, were heard and answered by God. He returned to the faith and spent the rest of his earthly life doing God's work, as a Bishop, in his prayer, and in his writings.
A phrase for which St. Augustine is well known, is found in Ch.1 of his 'Confessions':
'.. because you have made us
and drawn us to yourself,
and our hearts are restless
until they rest in you.'
So, a 'resting' Catholic could also be................