Saint Thomas Aquinas
For just as the natural body is one, though made up of diverse members, so the whole Church, Christ's Mystical Body, is reckoned as one person with its Head, which is Christ ... to the the Head of the whole Church all are united as members to a head: they constitute one mystical person.
The Second Coming of Christ must be dreaded by all who do not believe.
The Lord, therefore, knowing that there would be false teachers, warns of the various heresies to come by saying: "Beware of false prophets" (Matthew 7:15). He is clearly speaking of heretical teachers.
The sin of unbelief is greater than any sin which occurs in the perversion of morals.
Infidels of this sort are not damned for the sin of infidelity, but are indeed damned for other sins which cannot be remitted without faith. Wherefore, the Lord says: "If I had not come and spoken to them, they would not have sin" (John 15:22); that is, as St. Augustine explains it, "they would not have the sin of not believing in Christ."
Acceptance of the faith is a matter of the will, whereas keeping the faith, when one has once received it, is a matter of obligation. Wherefore, heretics should be compelled to keep the faith.
The souls of men attain to rest after death only by merits of faith. Therefore, no man can obtain the happiness of Heaven, which is the true knowledge of God, unless he first knows Him here on earth by faith.
There are two ways a man may deviate from the rectitude of Christian faith: first, because he is unwilling to assent to Christ ... this belongs to the unbelief in pagans and Jews. Secondly, because, although he intends to assent to Christ ... he chooses not what Christ really taught, but rather the suggestions of his own mind. Therefore, heresy is a species of unbelief.
For this reason the saints do not pray for unbelievers and wicked men, knowing them to be already condemned to eternal punishment.
There is no entering into salvation outside the Church, just as in the time of the Flood there was no salvation outside the Ark which denotes the Church.
The reality of the Blessed Sacrament is the unity of the Mystical Body, outside which there can be no salvation; for there is no entering into salvation outside the Church.
It is impossible for one who has a false opinion of God to know Him in any way at all... Therefore, unbelief is the greatest of sins ...The unbelief of heretics, who confess their belief in the Gospel and resist that faith by corrupting it, is a more grievous sin than that of the Jews who have never accepted the Gospel. The unbelief of a heretic is a more grievous sin than that of the heathens, because the heathens have not accepted the faith in any way at all. Hence, the unbelief of heretics is the worst sin. Speaking absolutely, the unbelief of heretics is worse than pagans or Jews.
To reject only one article of faith taught by the Church is enough to destroy the faith, just as one mortal sin is enough to destroy charity.
Just as "heresy" is so called from its being a "choosing," from the Greek word meaning "to cut off," so does the word "sect" derive its name from its being a "cutting off." Wherefore, any heresy and sect are the same thing, and each belongs to the works of the flesh.