As regards those who, by sin, have fallen from the received grace of Justification, they may be again justified, when, God exciting them, through the sacrament of Penance they shall have attained to the recovery, by the merit of Christ, of the grace lost. [Emphasis added]
God does not leave those who may fall without hope. By his grace in Christ they may be restored to fellowship with him.
There are some points regarding this chapter that are worth taking note of. First: we are restored to God through the Sacrament of Penance or Confession (now commonly called the Sacrament of Reconciliation):
For, on behalf of those who fall into sins after baptism, Christ Jesus instituted the sacrament of Penance, when He said, Receive ye the Holy Ghost, whose sins you shall forgive, they are forgiven them, and whose sins you shall retain, they are retained.
The mere act on our part of confessing our sins does not merit our forgiveness, as this passage makes clear. Rather, our sins are forgiven by Christ in the sacrament.
Secondly, the sacrament of reconciliation remits the guilt and eternal punishment of our sins, but not necessarily all the temporal punishment – for which we resort various penitential acts:
[H]ence it is to be taught, that the penitence of a Christian, after his fall, is very different from that at (his) baptism; and that therein are included not only a cessation from sins, and a detestation thereof, or, a contrite and humble heart, but also the sacramental confession of the said sins,-at least in desire, and to be made in its season,-and sacerdotal absolution; and likewise satisfaction by fasts, alms, prayers, and the other pious exercises of a spiritual life; not indeed for the eternal punishment,-which is, together with the guilt, remitted, either by the sacrament, or by the desire of the sacrament,-but for the temporal punishment, which, as the sacred writings teach, is not always wholly remitted, as is done in baptism, to those who, ungrateful to the grace of God which they have received, have grieved the Holy Spirit, and have not feared to violate the temple of God.
Finally, it's important to point out that not the sacrament only but even the desire for it may be sufficient to receive the grace promised in it, as seen in this passage. This should not be a comfort for the man who delays the day of his repentance, but for those who through no fault of their own are unable to receive the sacramental absolution, though they do in fact desire it. So much for the silly claims of those who wrongly suppose that an innocent failure to receive a sacrament is worthy of punishment. Here again we see that it's not what we do that matters: rather, it is what God does.