Liberalism is a mortal sin. But Catholic theology teaches us that all sins are not equally grave, that there is even a distinction of degree in venial sins. There are also degrees in the category of mortal sin, just as there are in the category of meritorious works. The gravity of sin is determined by the object at which it strikes. Blasphemy, for instance, which directly attacks God Himself, is a sin of much graver character than theft, which directly attacks man. With the exception of formal hate against God, which constitutes the deadliest of all sins and of which the creature is rarely culpable-----unless he be in Hell-----the gravest of all sins are those against faith. The reason is evident. Faith is the foundation of the supernatural order, and sin is sin in so far as it attacks this supernatural order at this or the other point; hence that is the greatest sin which attacks this order at its very foundations. To destroy the foundations is to destroy the entire superstructure. To cut off the branch of a tree will not kill it; but to lay the ax to the trunk or the roots is fatal to its life. Henceforth it bears neither blossom nor fruit. St. Augustine, cited by St. Thomas, characterizes sin against faith in these words: Hoc est peccatum quo tenentur cuncta peccata. "This the sin which comprehends all other sins."
The Angel of the Schools [St. Thomas Aquinas] expresses himself with his usual clearness on this point: "The gravity of sin is determined by the interval which it places between man and God; now sin against faith, divides man from God as far as possible, since it deprives him of the true knowledge of God; it therefore follows that sin against faith is the greatest of all sins."
When sin against faith is simply a culpable privation of the knowledge of God, it has not the same gravity as a direct and formal attack upon dogmas expressly defined by Divine Revelation. In this latter case sin against faith, so grave in itself, acquires that degree of gravity which constitutes heresy. It then contains all the malice of infidelity, and becomes an express protestation against the teachings of faith or an express adhesion to a teaching which is condemned as false and erroneous by the faith itself. Besides the deadly sin against faith itself, it is accompanied by hardness of heart, obstinacy, and the proud preference for one's own reason over the reason of God Himself.
Hence heretical doctrines-----and works inspired by them, constitute the greatest of all sins with the exception of the formal hate against God, of which only the demons in Hell and the damned are capable. Liberalism then, which is heresy, and all the works of Liberalism, which are heretical works, are the gravest sins known in the code of the Christian law.
Liberalism is, therefore, a greater sin than blasphemy, theft, adultery, homicide, or any other violation of the law of God, save in such case as where one acts in good faith, in ignorance, or without thought.
It is true that modern naturalism does not so regard or understand the case. But the law of the Church in matters of morals and doctrine is unchangeable; it ordains today as it did yesterday, and heresy is always heresy no matter what the shape it takes. Appearances may be fair, and the devil may present himself as an Angel of light. The danger is the greater as the outward show is more seductive.
Heresy has never been so insidious as under its present form of Liberalism. Its range is so wide that it touches upon every note in the scale, and finds an easy disguise in its protean facilities. But its most fatal shaft is in its plea for "liberty of mind." This in its own eyes is its cardinal virtue. "Intellectual freedom from dogmatism" is its boast, a boast in reality the mask of ignorance and pride. To meet such an enemy requires no ordinary courage guarded by a sleepless vigilance. When encountered it is obligatory upon the Catholic conscience to resist it with all the powers of the soul. Heresy and all its works are sins; Liberalism is the root of heresy, the tree of evil in whose branches all the harpies of infidelity find ample shelter; it is today the evil of all evils.