Objection 1. It would seem that a man cannot sin first of all against the Holy Ghost, without having previously committed other sins. For the natural order requires that one should be moved to perfection from imperfection. This is evident as regards good things, according to Proverbs 4:18: "The path of the just, as a shining light, goeth forwards and increases even to perfect day." Now, in evil things, the perfect is the greatest evil, as the Philosopher states (Metaph. v, text. 21). Since then the sin against the Holy Ghost is the most grievous sin, it seems that man comes to commit this sin through committing lesser sins.
Objection 2. Further, to sin against the Holy Ghost is to sin through certain malice, or through choice. Now man cannot do this until he has sinned many times; for the Philosopher says (Ethic. v, 6,9) that "although a man is able to do unjust deeds, yet he cannot all at once do them as an unjust man does," viz. from choice. Therefore it seems that the sin against the Holy Ghost cannot be committed except after other sins.
Objection 3. Further, repentance and impenitence are about the same object.
But there is no repentance, except about past sins. Therefore the same applies to impenitence which is a species of the sin against the Holy Ghost. Therefore the sin against the Holy Ghost presupposes other sins.
On the contrary, "It is easy in the eyes of God on a sudden to make a poor man rich" (Sirach 11:23). Therefore, conversely, it is possible for a man, according to the malice of the devil who tempts him, to be led to commit the most grievous of sins which is that against the Holy Ghost.
I answer that, As stated above (Article 1), in one way, to sin against the Holy Ghost is to sin through certain malice. Now one may sin through certain malice in two ways, as stated in the same place: first, through the inclination of a habit; but this is not, properly speaking, to sin against the Holy Ghost, nor does a man come to commit this sin all at once, in as much as sinful acts must precede so as to cause the habit that induces to sin. Secondly, one may sin through certain malice, by contemptuously rejecting the things whereby a man is withdrawn from sin. This is, properly speaking, to sin against the Holy Ghost, as stated above (Article 1); and this also, for the most part, presupposes other sins, for it is written (Proverbs 18:3) that "the wicked man, when he is come into the depth of sins, contemneth."
Nevertheless it is possible for a man, in his first sinful act, to sin against the Holy Ghost by contempt, both on account of his free-will, and on account of the many previous dispositions, or again, through being vehemently moved to evil, while but feebly attached to good. Hence never or scarcely ever does it happen that the perfect sin all at once against the Holy Ghost: wherefore Origen says (Peri Archon. i, 3): "I do not think that anyone who stands on the highest step of perfection, can fail or fall suddenly; this can only happen by degrees and bit by bit."
The same applies, if the sin against the Holy Ghost be taken literally for blasphemy against the Holy Ghost. For such blasphemy as Our Lord speaks of, always proceeds from contemptuous malice.
If, however, with Augustine (De Verb. Dom., Serm. lxxi) we understand the sin against the Holy Ghost to denote final impenitence, it does not regard the question in point, because this sin against the Holy Ghost requires persistence in sin until the end of life.
Reply to Objection 1. Movement both in good and in evil is made, for the most part, from imperfect to perfect, according as man progresses in good or evil: and yet in both cases, one man can begin from a greater (good or evil) than another man does. Consequently, that from which a man begins can be perfect in good or evil according to its genus, although it may be imperfect as regards the series of good or evil actions whereby a man progresses in good or evil.
Reply to Objection 2. This argument considers the sin which is committed through certain malice, when it proceeds from the inclination of a habit.
Reply to Objection 3. If by impenitence we understand with Augustine (De Verb. Dom., Serm. lxxi) persistence in sin until the end, it is clear that it presupposes sin, just as repentance does. If, however, we take it for habitual impenitence, in which sense it is a sin against the Holy Ghost, it is evident that it can precede sin: for it is possible for a man who has never sinned to have the purpose either of repenting or of not repenting, if he should happen to sin.
Source: Summa Theologica (New Advent)