God is said to be in a thing in two ways; in one way after the manner of an efficient cause; and thus He is in all things created by Him; in another way he is in things as the object of operation is in the operator; and this is proper to the operations of the soul, according as the thing known is in the one who knows; and the thing desired in the one desiring. In this second way God is especially in the rational creature which knows and loves Him actually or habitually. And because the rational creature possesses this prerogative by grace, as will be shown later (q.43 a.3). He is said to be thus in the saints by grace. But how He is in other things created by Him, may be considered from human affairs. A king, for example, is said to be in the whole kingdom by his power, although he is not everywhere present. Again a thing is said to be by its presence in other things which are subject to its inspection; as things in a house are said to be present to anyone, who nevertheless may not be in substance in every part of the house. Lastly, a thing is said to be by way of substance or essence in that place in which its substance may be. […] Therefore, God is in all things by His power, inasmuch as all things are subject to His power; He is by His presence in all things, as all things are bare and open to His eyes; He is in all things by His essence, inasmuch as He is present to all as the cause of their being. (Saint Thomas Aquinas. Summa Theologica I, q. 8, a. 3)

But what causes great amazement is the fact that God, although he is entirely in every place, does not inhabit in all men. In fact, my above-mentioned citation of the Apostle, or this one: ‘Do you not know that you are God’s temple and that God’s Spirit dwells in you?’ (1Cor 3:16) may not in fact be applied to all. That is why, on the other hand, the same Apostle said regarding others: ‘Anyone who does not have the Spirit of Christ does not belong to him’ (Rom 8:9). Furthermore, who would dare to think, except one who is entirely ignorant of the indivisibility of the Trinity, that the Father and the Son can inhabit someone in whom in the Holy Spirit does not dwell? Or that the Holy Spirit may live in someone in whom Father and the Son do not? In this way, one must admit that God is in all places by the presence of the divinity, but not in all places by the grace with which He inhabits souls. (Saint Augustine of Hippo. Epistle 187: Treatise on the presence of God, no. 16)

Well then, God who is in every place, does not, however, inhabit all; nor does He live in the same manner in all of those He inhabits. […] Then it must be said that those are far from Him, who due to sin have become totally different from Him; and that those are close to Him, who with a holy life receive His similitude, in the same way that one justly says that eyes are so much farther from the light of the earth, as they are blinder.  (Saint Augustine of Hippo. Epistle 187: Treatise on the presence of God, no. 17)

Having, therefore, been thus justified and having been made the ‘friends of God’ and ‘his domestics’ (Jn 15:15; Eph 2:19), ‘advancing from virtue to virtue’ (Ps 83:8), ‘they are renewed’ (as the Apostle says) ‘from day to day’ (2Cor 4:16), that is, by mortifying the members of their flesh (Col 3:5), and by ‘presenting them as instruments of justice’ (Rom 6:13,19), unto sanctification through the observance of the commandments of God and of the Church. (Denzinger-Hünermann 1535, Paul III, Council of Trent, Session VI, Decree on Justification, January 13, 1547)

Justification itself follows this disposition or preparation, which is not merely remission of sins [can. 11], but also the sanctification and renewal of the interior man through the voluntary reception of the grace and gifts, whereby an unjust man becomes a just man, and from being an enemy becomes a friend, that he may be ‘an heir according to hope of life everlasting’ (Tt 3:7). (Denzinger-Hünermann 1528, Paul III, Council of Trent, Session VI, Decree on Justification, January 13, 1547)

For they who are the sons of God, love Christ: ‘but they who love him, (as He Himself testifies) keep his words’ (Jn 14:23), which indeed with the divine help they can do. (Denzinger-Hünermann 1536, Paul III, Council of Trent, session VI, Decree on Justification, January 13, 1547)

All mortal sins, even those of thought, make men children of wrath (Eph 2:3) and enemies of God. (Denzinger-Hünermann 1680, Julius III – Council of Trent, Session XIV)

When, furthermore, we consider man’s personal dignity from the standpoint of divine revelation, inevitably our estimate of it is incomparably increased. Men have been ransomed by the blood of Jesus Christ. Grace has made them sons and friends of God, and heirs to eternal glory. (John XXIII. Encyclical Pacem in terris, no. 10, April 10, 1963)

We cannot be united with God unless we freely choose to love him. But we cannot love God if we sin gravely against him, against our neighbor or against ourselves: ‘He who does not love remains in death. Anyone who hates his brother is a murderer, and you know that no murderer has eternal life abiding in him’ (Jn 3:14-15). Our Lord warns us that we shall be separated from him if we fail to meet the serious needs of the poor and the little ones who are his brethren (Mt 25:31-46). (Catechism of the Catholic Church, no. 1033)

There can be people who have totally destroyed their desire for truth and readiness to love, people n everything has become a lie, people who have lived for hatred and have suppressed all love within themselves. This is a terrifying thought, but alarming profiles of this type can be seen in certain figures of our own history. In such people all would be beyond remedy and the destruction of good would be irrevocable: this is what we mean by the word Hell. (Benedict XVI. Encyclical Spe salvi, no. 45, November 30, 2007)