Grace, in short, is a gratuitous gift from God that is necessary for man to attain salvation. As St. Thomas states, “we must presuppose a gratuitous gift of God, Who moves the soul inwardly or inspires the good wish…that they are turned to God can only spring from God’s having turned them.” Since man has free will, he can choose to reject God’s gift, but he cannot do anything but receive that which God gives — man cannot merit grace or justification.
Now the effects of grace can be divided into justification (operating grace) and merit (cooperating grace). Justification is an effect not attributed to man, but God alone. Justification is one of the effects of habitual grace in that it makes our souls pleasing to God and “refers to a rightness of order within the person himself.” Since the fall of man, even justified man suffers from the effects of Original Sin (e.g. concupiscence, ignorance), but he is graced with forgiveness of sins and the indwelling of the Holy Spirit. This is seen from St. Thomas as he explains the effects of justification are the remission of sins and the infusion of grace. The Catechism further explains this as “justification is not only the remission of sins, but also the sanctification and renewal of the inner man.”
There are four aspects of justification: “the infusion of grace from God the mover, the movement of free choice to God from the one moved, the movement of free choice rejecting sin which is also moved by God, and the forgiveness of sins itself which is the termination of the movement of justification.”
The Catholic faith teaches that justification brings about a qualitative change in the soul and “there is no actual power in which man can attain grace.” Grace is freely given by God and not earned. If grace was earned condignly, then man would be equal to God and grace would necessarily be due to man. Further, grace creates a new quality in the soul —an accidental quality which means that it can only exist in another being, not substantially. Man remains man, but he has the quality of sanctifying grace — health of the soul.
Merit, cooperating grace, is due to the double act in man, that is, man’s will responds to God who moves. St. Augustine summed it thusly, “He operates that we may will; and when we will, He co-operates that we may perfect.” Further, habitual grace can be meritorious “in as much as it is the principle of meritorious works, which spring from free-will.” It must be kept in mind, however, that man doesn’t merit the first grace for it was Christ, “a divine person with a human nature, [who] merits the first grace for us by condign merit because He is the Word made flesh.”
When merit is spoken of, it does not mean merit in the sense of “earn” as in a worker merits his wage. Instead, it implies cooperating or congruent grace. Through this effect of sanctifying grace man does “merit”. This merit is not like that of equal to equal (condign), but is like a master showing love of his inferior servant. Cooperating grace allows that, “By God’s Will…the salvation of the human being will be given not only according to God’s part in the action, but will also proportionately correspond to mans’ end.” Said in another way, “No one merits grace or justification; but they merit the reward for a life lived in freedom as a result of being changed continuously by divine love: heaven.” Further, in each Christian act, there are two participants — God (The Holy Spirit) and man. The Holy Spirit’s part is rewarded condignly since He is God. Man’s part is rewarded congruently due to his proportionate participation; we are rewarded proportionately for how much we love God and respond to His Love. Also, it is important to briefly note that “by congruent merit, one can merit the first grace of conversion for another” because God loves those in union with Him and wants to love those His friends love. Their free-will is not lost in this for they must still be open to His conversion of them. Examples of this are St. Paul (St. Stephen’s prayer) and St. Augustine (St. Monica’s prayers).
Now, there are two false views on grace, Pelagianism and Protestantism, which help to explain what justification and merit are not. Protestants believe that there is not an ontological change caused by justification. Following Luther, they promote the false view that justification means God ignores the sin, and the sin remains. Man remains a capital murderer guilty of capital punishment, but God just looks away. Pelagianism, in the other extreme, believes that man could merit justification by his own power. Grace makes justification easier, but man doesn’t need grace to be justified. This is arguably even worse than the Protestant viewpoint.
With all of this in mind, especially the examination of merit, it is important to remember that while there is indeed a hierarchy in heaven and one can, in regards to the subject, receive more or less grace leading one to be more or less perfectly enlightened, there are not different holinesses in the Church. A priest or religious is not called to a different holiness than a married person — both are called to imitate Christ as is proper to their state in life. Both receive God’s gift of grace and are justified. Both can merit eternal life congruently. As St. Paul teaches, each ought to seek perfection in their vocation and Lumen Gentium confirms this when it states, “All Christians in any state or walk of life are called to the fullness of Christian life and to the perfection of Charity” in accord with the commandment “Be perfect, as your heavenly Father is perfect.”
 St. Thomas Aquinas. Summa I-II, Q109, A6.
 cf. St. Thomas Aquinas, Summa I-II, Q111, A2
 Fr. Mullady. Lesson 11: Justification – God Works in Us. Holy Apostles College and Seminary. 2017.
 St. Thomas Aquinas. Summa I-II, Q113, A1
 St. Thomas Aquinas. Summa I-II, Q113, A2
 CCC par. 1989.
 Fr. Mullady. Lesson 11. HACS. 2017.
 St. Thomas Aquinas. Summa I-II, Q111, A2.
 St. Thomas Aquinas. Summa I-II, Q111, A2.
 Fr. Mullady. Lesson 12: Merit – God Works with Us. HACS. 2017.
 Fr. Mullady. Lesson 12.HACS. 2017.
 Fr. Mullady. Lesson 12. HACS 2017.
 St. Thomas Aquinas. Summa I-II, Q112, A4.
 cf. Rom 12:6-8
 CCC. par. 2013.
 Matt. 5:48