I believe that many historians gave up trying to find ties between the Catholic Church and the assassination because there was nothing solid. One gentleman told me that Pope Pius IX had enough to deal with in protecting his church from the strife swirling around him in Italy at the time.
I can't remember if Andy Jampoler included thoughts on this in his The Last Lincoln Conspirator, and there is also a very good book by Kenneth Zanca on the subject of the church and Mary Surratt. Both men have been speakers at Surratt conferences and both like to share information - when they have time. I know that Andy is already working on another book, which is not assassination oriented.
I should have explained I'm not persuaded Pope Pius IX or the Jesuits ordered the assassination, but rumors of such and the Catholic affiliation of several conspirators really fueled interest in that at the time and that is interesting and forgotten. Charles Chiniquy, a defrocked Catholic priest of uncertain reliability authored a fast selling book about the Catholic conspiracy behind the Civil War and the assassination. Steers, in Blood on the Moon, seems not to have any interest the religious identity of the conspirators.