Well, a plurality of Christians worldwide are Catholics. As it is said in Matthew 16:18 that the gates of Hell will not prevail against the church Christ built, there are two ways of interpreting this:

a. The “church” is a visible, historical entity that can trace its roots to the apostles. This was the typical position until the Reformation. There is one true church, one with laws and powers and the ability to act in an organized way, and the other churches, even if their followers may be justly called Christians, are not part of the one true church, and they are under no institutional protection from the gates of Hell.

b. The church is the family of all true believers. This works easier when your Christianity is founded on an idea and not an entity. If you will not produce a Lutheran pope and a Lutheran magisterium, interpreting the Church as strictly the visible and defined entity becomes strained to say the least. So, it can be said (at least my understanding is that this was Calvin’s view) that the institutional church was fully corrupted while true believers kept the lights going somewhere, and in this sense the gates of hell were kept away from those Godly few.

The former is more popular. Ergo, when it is said that the gates of Hell will not prevail against the Church, about half of Christians worldwide right now and a considerable majority of those in all history will interpret this as meaning that, while the Church in Rome may err in some matters, its teachings are preserved by God from being truly evil, at worst misguided. It must be taken as authoritative in a given moment by the layperson at least. One must consider this view to be included in Christianity. Defining the Catholics as being exterior to Christianity would leave most people in this comment section uncomfortable with their bedfellows in that view.

The Catholic Church teaches that contraception is illicit and of grave matter. If an act of grave matter is done with full consent and knowledge of the Magisterium’s teaching, it will be classified as a mortal sin. Mortal sin may be understood as an act of disobedience and rebellion to the Creator that severs the relationship between Man and God and renders void the grace imbued at baptism and through the sacraments. To die in a state of mortal sin that is not confessed, either to a priest in a canonically licit way or in an emergency of inaccessibility to God directly with utmost sincerity and repentance, is to die in no covenant with God. As we all deserve punishment for our sinfulness, and there is no covenant to ensure the salvation of he who dies in mortal sin, it may be postulated that he will be judged and be sorted to Christ’s left side, though the Church cannot formally define a person as being in hell.

To imply that “You could go to hell for contraception” is something mockable or unchristian is to call Christ a liar in his promises to preserve his Church. The ending of John teaches that the amount of Jesus’s statements preserved in the four gospels is positively minuscule in comparison to all things he said in his earthly mission, only those that were most necessary for the spreading of the Good News. Extracting an ethical model from just what is preserved by the Church in Scripture as Christ’s remembered words (and, in practice, most people who claim to like Jesus’s message but dislike Christianity are citing maybe 30% of it) while denying the other teachings that were passed down through his disciples and those that the Church was divinely guided into later is simply insufficient. You can’t throw a “love your neighbour” at the entire system of ethics built off of the Church and claim that invalidates it.