Because the Catholic Church is Christ's Mystical Body, Our Lord in Acts 9:4 asked the nemesis of early Jewish converts, Saul of Tarsus, "Why do you persecute me?"
This intimate union is owing to the fact that the Church is Christ's mystical bride. In Ephesians 5:28–30, the former Saul of Tarsus explains, "He that loves his wife, loves himself ... as also Christ does the Church: because we are members of His body." This marriage, in turn, is owing to the Incarnation of Christ, which extends to His Church where the ordained priests act in persona Christi and the baptized are called the adopted children of God.
Because of this incarnational aspect of Christ's Mystical Body, the Church is immutable, meaning unchanging. This leads to the fact that She, the Bride of Christ, is indefectible. This means that Christ's truth and grace will never and can never fail universally in His Church.
The Catholic Church is in substance and essence the same Church that Christ founded and will remain so until the end of time. His Mystical Body, of which He's the head, will continue to be the source of truth and the means of grace for all generations. Leaders in the Church may betray their office like Judas but, nevertheless, Christ's promise uttered in Matthew 16:18 that "the gates of Hell shall not prevail against it" will never fail.
Watch the panel discuss the dangers of false religions in Friday's Download—The One True Faith Revisited: Jesus Christ. This week's topics on The Download include porn crisis and gay pride, among others.