It depends why they believe it. Someone of good will who loves God above all things and seeks to humble himself and abandon his will to God’s way, truth, and life–but is innocently mistaken about these things–maybe saved. Only God can search hearts and decide who has faith in Him and who is choosing a different way (either by obstinance, pride, malice, negligence, conveneince, to be favored by men, etc., etc.)
We’re not Gnostics. Remember, when Christ was born the angels brought glad tidings to men of good will, not men of perfect knowledge. Good will seeks to unite itself with God’s will. That will is manifested in Christ, the Scriptures, and the faith and sacramental life of the Catholic Church and knowledge of it helps us to more faithfully seek it and be strengthened in grace.
It’s our job to exhort all men to place their faith in God rather than themselves, to repent and adhere with faith and love to Christ, the way, the truth, and the life as a member of His Body, the Catholic Church. The Church has had great missionaries to pagans , heretics and schismatics, and also to Catholics (Sts. Francis of Assisi and Anthony Mary Claret come to mind). We are always in need of constant conversion and greater and greater faithfulness to God’s will. But, we leave the judgment to the Just Judge.
