Rat poison is 99.6% great food for rats! But it is the .4% poison that kills them stone dead.
The RCC has many good, sound, Scriptural doctrines, that many in the Church are genuinely saved. I have no doubt about that. I am not anti-Catholic. My wife was brought up and educated Catholic. I went on a site that contained the Hail Mary and other Catholic prayers, and she could recite them from memory! I heard her aunt sing "Ave Maria" beautifully, and she has a cousin who is a Catholic priest.
But, the RCC has certain doctrines that are the equivalent to rat poison for Christian believers - prayers to Mary and the Saints, worshiping the container that contains the wafer, kneeling and worshiping statues of Mary and Christ, the pagan Mass, confessing to a mortal priest instead of directly to God, faith plus works, observing the authority of the pope and following his teaching instead of listening to the indwelling Holy Spirit. Because these doctrines are killers that poison the faith of true believers, then I am in opposition to them, and can have no fellowship with those doctrines.