Objection 1. It would seem that no works of man are necessary that he may obtain Happiness from God. For since God is an agent of infinite power, He requires before acting, neither matter, nor disposition of matter, but can forthwith produce the whole effect. But man's works, since they are not required for Happiness, as the efficient cause thereof, as stated above (Article 6), can be required only as dispositions thereto. Therefore God who does not require dispositions before acting, bestows Happiness without any previous works.
Objection 2. Further, just as God is the immediate cause of Happiness, so is He the immediate cause of nature. But when God first established nature, He produced creatures without any previous disposition or action on the part of the creature, but made each one perfect forthwith in its species. Therefore it seems that He bestows Happiness on man without any previous works.
Objection 3. Further, the Apostle says (Romans 4:6) that Happiness is of the man "to whom God reputeth justice without works." Therefore no works of man are necessary for attaining Happiness.
On the contrary, It is written (John 13:17): "If you know these things, you shall be blessed if you do them." Therefore Happiness is obtained through works.
I answer that, Rectitude of the will, as stated above (Question 4, Article 4), is necessary for Happiness; since it is nothing else than the right order of the will to the last end; and it is therefore necessary for obtaining the end, just as the right disposition of matter, in order to receive the form. But this does not prove that any work of man need precede his Happiness: for God could make a will having a right tendency to the end, and at the same time attaining the end; just as sometimes He disposes matter and at the same time introduces the form. But the order of Divine wisdom demands that it should not be thus; for as is stated in De Coel. ii, 12, "of those things that have a natural capacity for the perfect good, one has it without movement, some by one movement, some by several." Now to possess the perfect good without movement, belongs to that which has it naturally: and to have Happiness naturally belongs to God alone. Therefore it belongs to God alone not to be moved towards Happiness by any previous operation. Now since Happiness surpasses every created nature, no pure creature can becomingly gain Happiness, without the movement of operation, whereby it tends thereto. But the angel, who is above man in the natural order, obtained it, according to the order of Divine wisdom, by one movement of a meritorious work, as was explained in the I, 62, 5; whereas man obtains it by many movements of works which are called merits. Wherefore also according to the Philosopher (Ethic. i, 9), happiness is the reward of works of virtue.
Reply to Objection 1. Works are necessary to man in order to gain Happiness; not on account of the insufficiency of the Divine power which bestows Happiness, but that the order in things be observed.
Reply to Objection 2. God produced the first creatures so that they are perfect forthwith, without any previous disposition or operation of the creature; because He instituted the first individuals of the various species, that through them nature might be propagated to their progeny. In like manner, because Happiness was to be bestowed on others through Christ, who is God and Man, "Who," according to Hebrews 2:10, "had brought many children into glory"; therefore, from the very beginning of His conception, His soul was happy, without any previous meritorious operation. But this is peculiar to Him: for Christ's merit avails baptized children for the gaining of Happiness, though they have no merits of their own; because by Baptism they are made members of Christ.
Reply to Objection 3. The Apostle is speaking of the Happiness of Hope, which is bestowed on us by sanctifying grace, which is not given on account of previous works. For grace is not a term of movement, as Happiness is; rather is it the principle of the movement that tends towards Happiness.
Source: Summa Theologica (New Advent)