Objection 1. It seems that disobedience is the most grievous of sins. For it is written (1 Samuel 15:23): "It is like the sin of witchcraft to rebel, and like the crime of idolatry to refuse to obey." But idolatry is the most grievous of sins, as stated above (Question 94, Article 3). Therefore disobedience is the most grievous of sins.
Objection 2. Further, the sin against the Holy Ghost is one that removes the obstacles of sin, as stated above (Question 14, Article 2). Now disobedience makes a man contemn a precept which, more than anything, prevents a man from sinning. Therefore disobedience is a sin against the Holy Ghost, and consequently is the most grievous of sins.
Objection 3. Further, the Apostle says (Romans 5:19) that "by the disobedience of one man, many were made sinners." Now the cause is seemingly greater than its effect. Therefore disobedience seems to be a more grievous sin than the others that are caused thereby.
On the contrary, Contempt of the commander is a more grievous sin than contempt of his command. Now some sins are against the very person of the commander, such as blasphemy and murder. Therefore disobedience is not the most grievous of sins.
I answer that, Not every disobedience is equally a sin: for one disobedience may be greater than another, in two ways. First, on the part of the superior commanding, since, although a man should take every care to obey each superior, yet it is a greater duty to obey a higher than a lower authority, in sign of which the command of a lower authority is set aside if it be contrary to the command of a higher authority. Consequently the higher the person who commands, the more grievous is it to disobey him: so that it is more grievous to disobey God than man. Secondly, on the part of the things commanded. For the person commanding does not equally desire the fulfilment of all his commands: since every such person desires above all the end, and that which is nearest to the end. Wherefore disobedience is the more grievous, according as the unfulfilled commandment is more in the intention of the person commanding. As to the commandments of God, it is evident that the greater the good commanded, the more grievous the disobedience of that commandment, because since God's will is essentially directed to the good, the greater the good the more does God wish it to be fulfilled. Consequently he that disobeys the commandment of the love of God sins more grievously than one who disobeys the commandment of the love of our neighbor. On the other hand, man's will is not always directed to the greater good: hence, when we are bound by a mere precept of man, a sin is more grievous, not through setting aside a greater good, but through setting aside that which is more in the intention of the person commanding.
Accordingly the various degrees of disobedience must correspond with the various degrees of precepts: because the disobedience in which there is contempt of God's precept, from the very nature of disobedience is more grievous than a sin committed against a man, apart from the latter being a disobedience to God. And I say this because whoever sins against his neighbor acts also against God's commandment. And if the divine precept be contemned in a yet graver matter, the sin is still more grievous. The disobedience that contains contempt of a man's precept is less grievous than the sin which contemns the man who made the precept, because reverence for the person commanding should give rise to reverence for his command. On like manner a sin that directly involves contempt of God, such as blasphemy, or the like, is more grievous (even if we mentally separate the disobedience from the sin) than would be a sin involving contempt of God's commandment alone.
Reply to Objection 1. This comparison of Samuel is one, not of equality but of likeness, because disobedience redounds to the contempt of God just as idolatry does, though the latter does so more.
Reply to Objection 2. Not every disobedience is sin against the Holy Ghost, but only that which obstinacy is added: for it is not the contempt of any obstacle to sin that constitutes sin against the Holy Ghost, else the contempt of any good would be a sin against the Holy Ghost, since any good may hinder a man from committing sin. The sin against the Holy Ghost consists in the contempt of those goods which lead directly to repentance and the remission of sins.
Reply to Objection 3. The first sin of our first parent, from which sin was transmitted to a men, was not disobedience considered as a special sin, but pride, from which then man proceeded to disobey. Hence the Apostle in these words seems to take disobedience in its relation to every sin.
Source: Summa Theologica (New Advent)