Every adult Protestant or Orthodox Christian has an objective grave moral obligation to convert to Catholicism. Protestantism is material heresy; every Protestant denomination teaches grave errors on faith and morals. All human persons are morally obligated to seek the truth on important matters of faith and morals. So to refuse to convert, once you have sufficient accurate knowledge of Catholicism, is an objective mortal sin.
But non-Catholic Christians can still be saved, without converting, if their refusal to convert is not an actual mortal sin. A sincerely mistaken conscience might reject membership in the Catholic Church without the full knowledge required for an actual mortal sin. A person who does not fully understand that a particular act is gravely immoral is not culpable for that act to the extent of actual mortal sin. So if the Protestant or Orthodox Christian does not realize that he should convert, then it is not an actual mortal sin, and he or she can be saved without converting to Catholicism.
Of course, any Christian can lose their salvation by sins in other areas of life. A Protestant who is not guilty of actual mortal sin for refusing to convert might still be guilty of actual mortal sin for adultery or hatred or theft or murder.
It is also more difficult for a Protestant Christian to be saved than for an Orthodox or Catholic, since the latter have the Sacrament of Confession to forgive mortal sins, whereas the Protestants must repent with perfect contrition if they have sinned by actual mortal sin.
Please take a look at this list of my books and booklets, and see if any topic interests you.