I'm perfectly willing to admit that Collins books are junk food, but this one wasn't even particularly tasty.
To begin with the entire plot centers around a Jesuit conspiracy to cause a rich nobleman to return an estate to the Catholic church that was seized by Henry the VIII. This ridiculous premise is unbacked by any explanation of why this estate is of any particular interest. This made the whole novel feel slack. Collins also sets up a race between the nobleman getting married and converting to Catholocism (the unlike route by which he is expected to give up the estate) but again there isn't much tension between the two as they don't really feel that mutually exclusive. So the lady wins the race with the aid of her friends but the nefarious Father Benwell doesn't give up but continues his scheming. Eventually he succeeds in driving a definitive albeit also ridiculous wedge between man and wife, getting the nobleman to renounce his marriage, sign over the property, and head to Rome to become a priest. At this point it turns out that the lady and her first husband really belong together but of course can't be. There is some handwringing over this until the nobleman conveniently dies but not before having his child throw the evil will in the fire foiling Father Benwell at the last moment. There's also a random daring rescue not described but used to kill time by the first husband while waiting for the nobleman to kick the bucket. This last item mentions a throw away connection with 'The Moonstone'. So much for the plot.
The characters are also quite weak. Father Benwell is a scheming Jesuit but not much of one. The whole plot to convert the nobleman doesn't have much deviousness to it. The weakness of the overall idea makes the Father seem sort of silly. He is supposedly quite important but doesn't really live up to the billing. The nobleman is perhaps the most interesting but still his primary characteristic as it's displayed to us is irritability. He is of course proud as well as racked by guilt over a duel he won. On the whole though he remains pretty mechanical, doing what Collins orders him too without much concern for real motivation.
Collins also seems afraid to give offense so he includes a sympathetic Jesuit without much function but a prominent place. He also tries at times to say things about how Father Benwell isn't really all bad. It just makes the story even more slack.
Lastly, there was some indecision about how to tell the story. Collins uses some of the standard tricks like diaries, letters, and written accounts. However most of the story is told by the usual omniscient narrator. There is no attempt to reconcile these two approaches.
Basically it's a failure on all scores.