Objection 1: It would seem that there is no certainty in the hope of a wayfarer. For hope resides in the will. But certainty pertains not to the will but to the intellect. Therefore there is no certainty in hope.
Obj. 2: Further, hope is based on grace and merits, as stated above (Q. 17, A. 1). Now it is impossible in this life to know for certain that we are in a state of grace, as stated above (I-II, Q. 112, A. 5). Therefore there is no certainty in the hope of a wayfarer.
Obj. 3: Further, there can be no certainty about that which may fail. Now many a hopeful wayfarer fails to obtain happiness. Therefore wayfarer's hope has no certainty.
On the contrary, "Hope is the certain expectation of future happiness," as the Master states (Sent. iii, D, 26): and this may be gathered from 2 Tim. 1:12, "I know Whom I have believed, and I am certain that He is able to keep that which I have committed to Him."
I answer that, Certainty is found in a thing in two ways, essentially and by participation. It is found essentially in the cognitive power; by participation in whatever is moved infallibly to its end by the cognitive power. In this way we say that nature works with certainty, since it is moved by the Divine intellect which moves everything with certainty to its end. In this way too, the moral virtues are said to work with greater certainty than art, in as much as, like a second nature, they are moved to their acts by the reason: and thus too, hope tends to its end with certainty, as though sharing in the certainty of faith which is in the cognitive faculty.
This suffices for the Reply to the First Objection.
Reply Obj. 2: Hope does not trust chiefly in grace already received, but on God's omnipotence and mercy, whereby even he that has not grace, can obtain it, so as to come to eternal life. Now whoever has faith is certain of God's omnipotence and mercy.
Reply Obj. 3: That some who have hope fail to obtain happiness, is due to a fault of the free will in placing the obstacle of sin, but not to any deficiency in God's power or mercy, in which hope places its trust. Hence this does not prejudice the certainty of hope.