It must be remarked that the choice in exercising our free will is itself a capacity that is given by God, and since its source is God, it is natural that its fulfilment, and therefore natural end is God, who is its cause. Hence, the proper exercise of the choice towards the perceived good, should be in keeping God’s law since God is the ultimate good.

Some quotes from Thomas Aquinas relating to free will:

“Free-will is the cause of its own movement, because by his free-will man moves himself to act. But it does not of necessity belong to liberty that what is free should be the first cause of itself, as neither for one thing to be cause of another need it be the first cause. God, therefore, is the first cause, Who moves causes both natural and voluntary. And just as by moving natural causes He does not prevent their acts being natural, so by moving voluntary causes He does not deprive their actions of being voluntary: but rather is He the cause of this very thing in them; for He operates in each thing according to its own nature.” (Summa Theologiae I.83)

“Besides, God not only gives powers to things but, beyond that, no thing can act by its own power unless it acts through His power, as we showed above. So, man cannot use the power of will that has been given him except in so far as he acts through the power of God. Now, the being through whose power the agent acts is the cause not only of the power, but also of the act. This is apparent in the case of an artist through whose power an instrument works, even though it does not get its own form from this artist, but is merely applied to action by this man. Therefore, God is for us the cause not only of our will, but also of our act of willing.” (Summa Contra Gentiles III.89.5)

Reading 1  (Sirach 15:15-20)

If you choose you can keep the commandments, they will save you;
if you trust in God, you too shall live;
he has set before you fire and water
to whichever you choose, stretch forth your hand.
Before man are life and death, good and evil,
whichever he chooses shall be given him.
Immense is the wisdom of the Lord;
he is mighty in power, and all-seeing.
The eyes of God are on those who fear him;
he understands man’s every deed.
No one does he command to act unjustly,
to none does he give license to sin.