This is a hard saying, but I have been thinking for over a year about the necessity of being a Catholic in order to experience the real love of God and be one with Him. One cannot judge others, but the nature of love is such that it is based on Truth and the fullness of Truth is in the Catholic Church. Being one with the teachings of the Church is like being of one mind with the Beloved, Christ.
If one disagrees with one item of orthodoxy, one is removed from unity with the Catholic Church and therefore, oneness with Christ.
All baptized people are children of God and heirs of heaven. All baptized people are in sanctifying grace. But, there is a catch. As soon as one is at an age of reason and is choosing to be a Protestant, this relationship is broken.
The stages of holiness I have been describing cannot be reached in the fullness without a basis of orthodoxy. Does this mean that no one outside the Church is a saint? Obviously, the Church does not canonize Protestants. That some people outside merit special grace is because of the merits of the Catholic Church. This is part of the mystery of grace.
No one sees God who is not purified of sin and self-will. This is our mission, to cooperate with grace in order to know, love and serve God in this world and praise Him in the next.