The grace of Salvation is indeed a free gift of God and something we cannot earn ourselves. Pure Pelagianism suggests that we do not need grace at all, and Semi-Pelagianism suggestst aht while we need Grace, we are somehow partly responsible for obtaining it (i.e. we earn the Grace, but we don’t earn salvation per se). Of course both are wrong and condemned as heresy.
However, the Church has always taught that God does not force Himself upon us, but that our wills remain free to reject Him. Thus, in order to attain Heaven, we must cooperate with God’s grace and not resist it. This cooperation is the source of the Catholic doctrine of merit and is a summary of the good work required for salvation.
In a more practical sense, we just need to look at the Church in the world. God has given us the sacrament of Baptism as a means of obtaining Sanctifying Grace. The Church freely offers this sacrament, but as adults, we have to choose to receive it. Even if we were baptised as children, we have to choose to avoid mortal sin and avail ourselves of confession if it occurs. Even these acts of receiving the sacraments involve God’s grace as an impetus to receive them, nevertheless, our free will must cooperate or else we can easily frustrate the action.
This is what is meant in the Salesian theory as a pre-vision of merit. Will we resist the grace that God is freely giving us, or will we allow it to transform us and sanctify us? God offers this grace to all (so the mere offering cannot be considered predestination). St. Thomas’s theory seems to imply that our free will decision is not an important component, and ultimately seems inconsistent to me.
While Thomas explicitly denies antecedent reprobation, it would almost seem logically necessary from his position on predestination. If God chooses who is predestined without conisderation of the cooperation of their will such that those who are predestined are not predestined in anyway because they choose to be, and if there is no other alternative to Heaven but hell, then it would seem that all who are not predestined are among the reprobate, yet God only wills people to be reprobate in light of their free-will decision. It is an apparent inconsistency (one which I am sure Thomists who agree would have some kind of counter argument for, but I have never heard one that convinced me).