Wow, you have just asked a MILLION dollar question! I am no theologian, but I will give you some sources I used to try to read about it and also the “bit” I “think” I understand.
Grace
"Sanctifying Grace is the Grace we receive through Baptism.
“St. Paul in his letter to the Romans asks “…are you unaware that we who were baptized into Christ Jesus were baptized into his death? We were indeed buried with him through baptism into death, so that, just as Christ was raised from the dead by the glory of the Father, we too might live in newness of life (Rom 6:3-4).” “This is why baptism is the vital first step in the life of a Christian. Also see (John 3:5).” Thus the cleansing waters of baptism wash away the inequities of sin and through the grace of God begin a sustaining relationship in Christ through the work of the Holy Spirit. From what I read, this means Sanctifying Grace, also called Habitual Grace, is a gift, once given through Baptism by God and is never taken away.” This does not mean we believe that once we are given this gift, all we need do to be saved is believe.
The Catholic Church does not teach once saved always saved.
Pretty sticky and a source of argument since Luther and maybe before. That little propostion I know nothing about.
http://www.tanbooks.com/doct/divine_grace.htm"What is Sanctifying Grace? It is none other than the life of God Himself imparted to the soul, whereby man is raised, in a true sense, to the divine level and by which man becomes a friend and a child of God, an heir to Heaven, and is thereby and thereafter pleasing in His sight—so long as man remains in the state of Sanctifying Grace… And how can we lose it? We lose it by committing mortal sins, which are those sins 1) of a grievous nature (in themselves, or which we think are grievous), 2) which we fully know to be mortally sinful, and yet 3) which we commit anyway, with the full consent of our will.
We also believe in what is called Actual Grace:Although Sanctifying Grace instills us with life, there are moments in our lives in which we are especially moved by God’s love. These graces help us to sustain our relationship with Christ and are called actual graces, because they reflect the intervention of God in our lives. Actual grace is what moves a person’s heart to conversion and penitence and sustains the soul in the process of justification and sanctification.
The seven sacraments of the Church are also a work of actual grace to nourish the Body of Christ and its members. sustaining life of grace in the Eucharist to his apostles, "I am the living bread that came down from heaven; Whoever eats this bread will live forever…(John 6:51)."Now I don’t know if by Justifying Grace you mean justification in which one is in a state of constant grace as long as one believes in Christ and His teachings, but I am going on the assumption that is what you mean.
The following is the definition of Catholic Justification:JUSTIFICATION, THEOLOGY OFThe process of a sinner becoming justified or made right with God.
As defined by the Council of Trent, “Justification is the change from the condition in which a person is born as a child of the first Adam into a state of grace and adoption among the children of God through the Second Adam, Jesus Christ our Savior” (Denzinger 1524). On the negative side, justification is a true removal of sin, and not merely having one’s sins ignored or no longer held against the sinner by God. On the positive side it is the supernatural sanctification and renewal of a person who thus becomes holy and pleasing to God and an heir of heaven. The Catholic Church identifies five elements of justification, which collectively define its full meaning. The primary purpose of justification is the honor of God and of Christ; its secondary purpose is the eternal life of mankind. The main efficient cause or agent is the mercy of God; the main instrumental cause is the sacrament of baptism, which is called the “sacrament of faith” to spell out the necessity of faith for salvation. And that which constitutes justification or its essence is the justice of God, “not by which He is just Himself, but by which He makes us just,” namely sanctifying grace. Depending on the sins from which a person is to be delivered, there are different kinds of justification. An infant is justified by baptism and the faith of the one who requests or confers the sacrament. Adults are justified for the first time either by personal faith, sorrow for sin and baptism, or by the perfect love of God, which is at least an implicit baptism of desire. Adults who have sinned gravely after being justified can receive justification by sacramental absolution or perfect contrition for their sins. (Etym. Latin justus, just + facere, to make, do: justificatio.)
Another of the differences between Catholic and Protestant belief in grace is that Catholics believe in an intermediary between themselves and Christ/God, the Priest, especially in the Sacrament of Reconciliation, or Confession. Another would probably be the necessity Catholics see for the Seven Sacraments, Baptism, Act of Reconciliation, Holy Eucharist, Confirmation, Blessing of the sick, or as it was known in my day, Extreme Unction (for the dying) Holy Orders, Matrimony. Hope this helps.
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)… you are, nonetheless, serving God… and doing what the bible says to do… It’s about obedience… obeying when you don’t feel like it…