Aquinas: an act can be praiseworthy and meritorious though the will be determined to one thing
So he teaches that a deed may deserve praise or blame, merit or demerit, simply because it is spontaneous, voluntary, and not forced, even though the will is fixed on that one thing.
On the third book of the Sentences (margin: dist. 12, q. 2, art. 1, ad 3) he writes: 'The inability that comes from compulsion, which is the opposite of the voluntary, removes merit and demerit. Not so the inability that comes from perfection in good or in evil. That does not remove the voluntary; it presupposes a will confirmed in one direction.'
In the same book (margin: dist. 18, art. 2, ad 5) he explains how Christ could merit by a free choice fixed on one single act, namely the love of God: 'Even if it were fixed on one single act, such as loving God, which it cannot fail to do, it does not on that account lose its freedom or its claim to praise or merit. It goes toward that act spontaneously and not under force, and so remains in command of its act.'
Latin
XVIII. Hinc est quod docet opus quoddam esse posse laude, vel vituperio dignum, meritorium, vel demeritorium, ex hoc quod est spontaneum non coactum & voluntarium, tametsi voluntas illius respectu sit ad unum determinata. Nam in tertium sententiarum scribens, [marg.: Dist. 12. q. 2. art. 1. ad 3.] Impotentia coactionis, inquit, quae opponitur voluntario, tollit rationem meriti & demeriti, non impotentia quae est ex perfectione in bonitate, vel malitia, quia hoc voluntarium non tollit, sed ponit voluntatem confirmatam ad unum. Et in eodem libro, ut explicaret quomodo Christus mereri potuerit per liberum arbitrium determinatum ad unum numero, scilicet ad diligendum Deum: [marg.: Dist. 18. art. 2. ad 5.] Dicendum, inquit, quod etiamsi esset determinatum ad unum numero sicut ad diligendum Deum, quod non facere non potest: tamen ex hoc, non amittit libertatem, aut rationem laudis, sive meriti, quia in illud non coacte, sed sponte tendit, & ita est actus sui Dominus.