We gain Heaven through faith in Christ...not the Roman Catholic church.
When a person follows Christ they are indwelt with the Holy Spirit to guide them. They have the Truth at that point. They are sanctified. They cannot be saved anymore...in other words they are fully saved. It's not an ongoing process. Christ doesn't save you a little today and then a little tomorrow.
The believer in Christ is fully saved.
We can follow Christ according to our own subjective interpretation of the Bible, and thus we can easily satisfy our hidden egoisms, without an authoritative guide here on earth. It is no secret that there are a multitude of contradictions between the thousands of independent protestant groups. Also if we say “once saved always saved”, then are we implying that tomorrow we no longer have a free will like an animal and so we can no longer sin, and thus we never again need the mercy of God and we never again need to repent since after “once saved” we are all perfect saints and angels and never sin again?!?!?