Thomas Aquinas, Compendium of Theology

The suggestion might be put forward that sins are unforgivable, not through any lack of power on God's part, but because divine justice has decided that anyone who falls from grace shall never more be restored to it. But such a position is clearly erroneous. There is no provision in the order of divine justice to the effect that, while a person is on the road, he should have assigned to him what belongs to the end of the journey. But unyielding adherence to good or to evil pertains to the end of life's course; immobility and cessation from activity are the terminus of movement. On the other hand, the whole of our present life is a time of wayfaring, as is shown by man's changeableness both in body and in soul. Accordingly divine justice does not determine that after sinning a man must remain immovably in the state of sin.

Moreover, divine benefits do not expose man to danger, particularly in affairs of supreme moment. But it would be dangerous for man, while leading a life subject to change, to accept grace if, after receiving grace, he could sin but could not again be restored to grace. This is so especially in view of the fact that sins preceding grace are remitted by the infusion of grace; and at times such sins are more grievous than those man commits after receiving grace. Therefore we may not hold that man's sins are unforgivable either before or after they are committed.