Greatest offense to Christ? In Acts 9:3-5, on the road to Damascus, Saul (later St. Paul) was essentially told by Christ, "Your persecution of My body, the church, is persecution of Me."
The greatest attack on the mystical body of Christ is not from without; it is from within. We know by the fruit (or lack of). Only one-third of Catholics believe in the Eucharist. As in John 6:66-67, many have departed for the false "outside" unity of ecumenism. The great apostasy? Zechariah 13:8-9?
Pope Pius X's "Pascendi," Pius XI's "Mortalium Animos" and Pius XII's "Mystic Corporis Christ" warned against modernism, novelty and any authority not given by Christ to His one mystical body, the Catholic Church.
Modernism - Progressivism proceeds under a blend of truth and error (which is really error). Truth demands totality - don't mix the true Catholic Chinese underground church with the false, counterfeit patriotic communist church.
Why would a Catholic seek understanding from a non-Catholic (C.S. Lewis, "Mere Christianity")? There is no more basic Christianity than the Eucharist (John 6) and its twin, confession (John 20:19-23 and I Corinthians 11:27-29).
Yet "Lumen Gentium" - Vatican II's basic decree on ecumenism - attempted to enlarge unity. This compromises the authority of baptism, marriages in respect to dispensations outside the true cement of the Eucharist in a nuptial Mass, and suggests that the orthodox can confect the Eucharist outside the primacy of Peter.
Pope Boniface VIII (Unam Sanctam) warned against anything outside the Catholic pontiff. Relevant passages: Matthew 16:17-19, Matthew 28:20, John 6, Ephesians 1:22-23, 3:10-11, 5:25-32, Colossians 1:24, I Timothy 3:15.
Confusion is not a manifestation of the Holy Ghost, who guides the church. Don't assume that the church is bad while John Paul II (who apologized for the church before his own scandal) is "great."
Greatness comes from not charismatic idolatry or innovation. Actually, John Paul's papacy was one of great decline in the most basic belief: the Eucharist and confession.