"“Being justified” means being made righteous, that is, being accepted by God’s merciful justice to enter into communion with him and, consequently, to be able to establish a far more genuine relationship with all our brethren: and this takes place on the basis of the complete forgiveness of our sins.
Well, Paul states with absolute clarity that this condition of life does not depend on our possible good works but on the pure grace of God: “[We] are justified by his grace as a gift, through the redemption which is in Christ Jesus” (Rom 3: 24). With these words St Paul expressed the fundamental content of his conversion, the new direction his life took as a result of his encounter with the Risen Christ."
“Following St Paul, we have seen that man is unable to “justify” himself with his own actions, but can only truly become “just” before God because God confers his “justice” upon him, uniting him to Christ his Son. And man obtains this union through faith. In this sense, St Paul tells us: not our deeds, but rather faith renders us “just”. This faith, however, is not a thought, an opinion, an idea. This faith is communion with Christ, which the Lord gives to us, and thus becomes life, becomes conformity with him. Or to use different words faith, if it is true, if it is real, becomes love, becomes charity, is expressed in charity. A faith without charity, without this fruit, would not be true faith. It would be a dead faith.”
“…we discovered two levels: that of the insignificance of our actions and of our deeds to achieve salvation, and that of “justification” through faith which produces the fruit of the Spirit. The confusion of these two levels has caused more than a few misunderstandings in Christianity over the course of centuries. In this context it is important that St Paul, in the same Letter to the Galatians radically accentuates, on the one hand, the freely given nature of justification that is not dependent on our works, but which at the same time also emphasizes the relationship between faith and charity, between faith and works: “In Christ Jesus neither circumcision nor uncircumcision counts for anything, but only faith working through love” (Gal 5: 6). Consequently, there are on the one hand “works of the flesh”, which are “immorality, impurity, licentiousness, idolatry…” (Gal 5: 19-20): all works that are contrary to the faith; on the other, there is the action of the Holy Spirit who nourishes Christian life, inspiring “love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control” (Gal 5: 22-23). These are the fruits of the Spirit that blossom from faith.”
For the fuller context see full texts of Pope Benedict XVI linked above.
Now can a person then --after God has “justified” him – loose that state via his seriously sinful actions or serious sinful omissions --his mortal sin that is? Yes.
We need to remain “living” in Christ and should we fall – we will need the blood of Christ to cleanse us of such and restore us to “justification”.