The author is asserting the Catholic doctrine on salvation which teaches that a man must be in a state of grace, that is friendship with God, at the moment of his death to enter Heaven.
Due to the sin of Adam we have lost the inheritance of sanctifying grace because he squandered it. Thus every man is born in a state of damnation and he remains so until he receives faith through baptism as indicated in John 3:36
“He who believes in the Son has eternal life, and he who disobeys the Son shall not see life but the wrath of God abides on him.”
Catholics can lose the state of grace by severing themselves from God by committing a mortal sin, which is a deliberate sin concerning grave matter, such as sins of the flesh, idolatry, heresy, schism, etc.
Even cherishing a single sinful thought is enough to merit Hell.
St. Alphonsus, On the Number of Sins Beyond Which God Pardons No More:
St. Gregory the Great relates that a child, five years old, who had
arrived at the use of reason, was seized by a Devil for having uttered
a blasphemy, and carried in the Hell. Another boy of eight died after his first sin, and was lost forever. The Holy Mother of God revealed to that great Servant of God, Benedicta of Florence, that a girl of twelve was damned after her first sin.
Because sin is an offense against God's infinite majesty, no punishment is severe enough, however, He mercifully offers us the graces and opportunities to repent.
Most Protestants reject this teaching and believe God forensically justifies, that is "covers" the sins of the person but does not erase them. This is problematic since God as perfect good cannot dwell with evil but he destroys it by his presence. As we see from 2 Samuel 6:6-7:
And when they came to the floor of Nachon, Oza put forth his hand to
the ark of God, and took hold of it: because the oxen kicked and made
it lean aside. And the indignation of the Lord was enkindled
against Oza, and he struck him for his rashness: and he died there
before the ark of God.