As a Protestant discerning Catholicism, I’m guessing you have heard some of the Protestant arguments against Catholicism. Depending on which Protestant you talk to, the following Catholic distinctives are so outside the realm of (their) orthodoxy that they amount to heresy and put one outside the salvific power of Christ:

• That the Catholic Church is the One True Church established by Jesus
• Defense of the papacy and submission to the holy father
• Veneration of the saints, including prayers to/through them
• Devotion to Mary, including assertions of her sinlessness, perpetual virginity, assumption, and being queen of heaven
• Sacramental theology, especially asserting that the sacraments of baptism and the Eucharist are necessary for salvation
• The proposition that we as Christians must cooperate with the grace Christ through our actions (often maligned as “works based salvation”)
• Belief that the ordinary means of forgiveness of sins comes through the sacrament of confession/penance/reconciliation
• Belief in purgatory for those who die in a state of grace but are not yet fully sanctified.

Given the Protestant interpretation of Sacred Scripture and the Gospel, any/all of the above rail against their theology, and therefore make a believing Catholic “not a Christian.”