"All that God does in time, He foresees and predestines from all eternity. He foresaw and predestined in detail all the means by which He had to inspire his followers with fidelity, obedience, and perseverance. That is what predestination means.
The benefit of this doctrine is to have us place our will and liberty in God's hands, asking Him to direct this will so that it may never stray from the right path, and thanking Him for all the good that it does and believing that He operates in it without weakening or destroying it, but on the contrary, elevating, strengthening, and granting it to make good use of itself, which is the most desirable of all good things.
We must not therefore, attribute the cause of salvation to him who wills or to him who runs, but to God who shows mercy. This means that neither their running nor their willing are the primary cause, and still less the only cause of their salvation; but this cause is the accompanying and preventing grace that gives them strength to continue until the end, and this grace does not act alone; for we must be faithful to it; and to accomplish this effect, it gives us the power to co-operate with it, so that we can say with St Paul: “Yet not I, but the grace of God with me”.
God is the author of all the good we do. It is He who brings it to completion, just as it is He who begins it. His Holy Spirit forms in our hearts the petitions He wishes to grant. He foresaw and predestined all that; for predestination is nothing else. In all this we must believe that no one perishes, no one is cast off as a reprobate, except through his own fault. If human reasoning finds a difficulty in this, and cannot reconcile all phases of this holy and inviolable doctrine, faith must continue to reconcile all things, at the same time waiting until God causes us to see everything in Him, the fountainhead.
The whole doctrine of predestination and grace may be summed up briefly in these words of the Prophet: “Destruction is thy own, O Israel; thy help is only in me. So it is. If we do not see the consistency in all this, it suffices for us that God knows, and we must humbly believe him. God's secret is his own."
Jacques-Bénigne Bossuet (quoted in “Predestination”, by Reginald Garrigou-Lagrange O.P.)