Of the eternal lot of such as, wilfully sinning against the light, reject the known truth of their Saviour, there never can he any doubt. The doubt and difficulty begins when we turn to others, who never have heard of Christ, or who, however much they have heard of Him, never seem to have gathered tidings sufficient and adequate to their minds. Their situation, to the Christian thinker who mixes in modern life and knows men, is one of the darkest regions of his theology. He can but fall back on his Lord's precept: Judge not (Matt. vii, 1).
On the dichotomy, Matt. xxv, 31-46, I quote Scripture Manuals for Catholic Schools, St Matthew, pp. 212-3: "But why is no mention made of faith, a condition of salvation upon which our Saviour and His apostles otherwise insist so strongly? Probably Lactantius and Origen are right in their conjecture, that though the Saviour of all is likewise judge of all (Acts xvii, 31), and will gather all nations (v. 32) before His judgement-seat, yet the sheep and goats here particularised are good and bad Christians. The heathen who contumaciously reject Christ are already judged and condemned (John iii, 18: Mark xvi, 16). Of the heathens to whom Christ has not been preached, we get no information here."
St Thomas himself habitually views the Christian in every man; and hardly conceives, still less can be considered here to discuss, the position of him who is not a Christian and a Catholic.
Of God and His Creatures: 3.160