Protestant here. Don’t shoot.

. . .
What is wrong with my understanding of this faith vs work debate? This is like, the biggest mountain for me in my understanding of Catholicism. Please explain!
Laudatur Iesus Christus.
The descriptions which have been given so far are useful. I would like to be a little more “schematic” in the hope of fitting the elements of the Catholic teaching together in a way that makes sense.
God is love. Sanctifying Grace is “the life of God.” Sin is the separation of man’s will from God’s will. Salvation is the union of man with God and his joining in the life of God – which is love within the Trinity.
Given these understandings, the key is to realize that salvation is an active and living thing and it has more than one part. The first part is justification: God the Father made each person and all of creation as gifts for the Son. To enter into the love that the Father has for the Son, one must cooperate in this gift. Baptism is the means of doing this. In Baptism a person freely gives himself or his children to Christ as a gift, in fulfillment of the Father’s intention in creation. This is why one is saved by Baptism – it is one’s entry into the life of the Trinity, a taking part directly in the Father’s love for the Son.
However, this is not all. The initial entry saves, because it allows one to participate in the eternal life of the Trinity – this is what man’s salvation is. But once given to Christ, we must turn again to the Father and participate in the Son’s love for the Father. This is done by acting in Christ, as part of His Mystical Body. This involves many acts of praise offered to the Father and work to extend salvation to the world – the work and will of Christ. The more we act in Christ the more we participate in the life and the liveliness of the Trinity, we have life “more abundantly.”
While acting, however, because we remain free, we can change our mind and choose to disobey, as Adam did. Since God respects our freedom, He allows this voluntary separation from His will. The separation occurs either by our declaring to Him by word or gesture that we have elected to act separate from Him; or by our taking action which is incompatible with His will. All of these are termed “mortal sins,” separations of our will from God’s to such an extent that we are no longer participating in the life of the Trinity, but have made ourselves autonomous from Him. That is sin that “kills the life of grace in us.”
Jesus has provided remedies for these lapses into sin, in the sacraments of Confession and Anointing of the Sick, both of which effect one’s re-entry into the flow of love between the persons of God.
Actions which we do on human initiative, though good – even identical to the acts done in Christ – do not save, because they represent expressions of human will alone – it is not the accomplishments of good works that saves, but the participation in the life of the Trinity – hence we say “grace alone saves.”
Further, once in a state of grace, we are still freely cooperating with the will of God, not being controlled by Him as slaves, but working with Him as adopted children. Hence, we must work to become better and better at our work in praising the Father and working with the Son. When we fall short in our cooperation, we sin – but not mortally – so long as we intend and are making an honest effort to do Christ’s will. These are easily forgiven sins – or as the Lord’s prayer calls them “shortfalls.”
Thus the Catholic view sees eternal life as a present and demanding reality. It is not only a matter of life after death, but abundant participation in the life of the Trinity in the present life – offering praise, suffering, working for good, teaching, offering comfort, healing – all as Jesus did when He walked the earth, and as He continues to do through His Sacraments, including His Church.
In this way, by doing His will and acting in His Mystical Body, each Baptized person not only imitates Christ as a model, but enters into Him and acts as a part of Him in accomplishing His work, which is to love the Father in the Spirit. Hence, by working in Christ as children of God we are saved – in the present tense – we are living the life of the Trinity, the love of each person of the Trinity for the others. This is holiness.
Hence, when Catholics read that “only one thing is required,” we do no take that to be one thing among a multitude of things. Rather we take it to mean that the one thing, grace, encompasses all other things, including and sanctifying all of human life and creation. It is not one thing among many others, it is one all encompassing thing, that God may be all in all.
Pax Christi nobiscum.
John Hiner