Dear Leon:

The Church teaches that when we are baptized, and again in Confirmation (and in Holy Orders), a spiritual and indelible mark or imprint or character is placed on our soul. This is defined explicitly by the Council of Trent (Sess. VII, can. ix). St. Cyril (Pręp. in Cat.) calls baptism a "holy and indelible seal", and Clement of Alexandria (De Div. Serv., xlii), "the seal of the Lord."

Thus, even if we go to hell, that character (imprint) of baptism, Confirmation, or Holy Orders (if we receive them) are still on our soul.

But, does the Holy Spirit remain with us when we are in a state of sin (mortal sin)?

Fr. Paul O'Sullivan, O.P, author of The Holy Ghost:
Our Greatest Friend (with ecclesiastic approval, 1952) and Devotion to the Holy Spirit (with imprimatur, 1950) states:

The Holy Spirit abides in our soul as long as we preserve ourselves from mortal sin. Mortal sin forces the Holy Spirit to depart. Light and darkness cannot dwell together. "What fellowship hath light with darkness?" asks St. Paul. (2 Cor. 6: 14) The Holy Spirit has no communication nor indwelling with the evil spirit; they cannot abide together in a soul, and it is certain that a soul while in mortal sin is given over to the power of the evil one. The wisdom of the Holy Spirit does not enter a wicked soul; the Holy Spirit hates hypocrisy and does not dwell in a body that serves sin and vice."

The Catechism states:

1855 Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him.

Venial sin allows charity to subsist, even though it offends and wounds it.

1856 Mortal sin, by attacking the vital principle within us - that is, charity - necessitates a new initiative of God's mercy and a conversion of heart which is normally accomplished within the setting of the sacrament of reconciliation.

1861 Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God's forgiveness, it causes exclusion from Christ's kingdom and the eternal death of hell...

While the Catechism uses different terminology, it says the same thing as Fr. O'Sullivan: Mortal sin destroys charity in one's heart. Charity (love) is God for God is love (1 John 4:7-8). As such, we must be restored to sanctifying grace (in which the Holy Spirit indwells us) by a new conversion of heart through an action of the Church (Sacrament of Confession).

Venial sin is what diminishes the presence of the Holy Spirit (grieves the Holy Spirit), and makes us less sensitive to His presence, but we retain sanctifying grace (and thus the Holy Spirit within us).

Mortal sin kills our souls, destroy santifying grace, and thus leaves us without the Holy Spirit.

Sacrament of Confession restores our soul (resurrects it) back to sanctifying grace and the presence of the Holy Spirit.

God Bless,
Bro. Ignatius Mary