What does it mean to be separated from the Church?

Commenting on the prodigal son’s travel to a far country, St. Ambrose says, “What is more remote than to leave one’s self, to be separated not by space, but by manners, from differing in tastes, not by countries, and the excesses of the world? For whoever separates himself from Christ is exiled from his country, is a citizen of the world. But we, ‘we are not strangers and passing, but we are citizens of the sanctuary, and of the house of God’ (Eph. 2:19); for ‘far removed as we were, we were brought together in the blood of Christ’ (Eph. 2:13). Let us not be evil toward those who come back from far off, since we too have been in a far country … in the shadow of death but were made alive in the shadow of Christ.”¹¹⁰³