uncleanness of a woman suffering from an issue of blood, says: "Every
sin is an uncleanness of the soul, but especially idolatry."

_I answer that,_ The gravity of a sin may be considered in two ways.
First, on the part of the sin itself, and thus idolatry is the most
grievous sin. For just as the most heinous crime in an earthly
commonwealth would seem to be for a man to give royal honor to
another than the true king, since, so far as he is concerned, he
disturbs the whole order of the commonwealth, so, in sins that are
committed against God, which indeed are the greater sins, the
greatest of all seems to be for a man to give God's honor to a
creature, since, so far as he is concerned, he sets up another God in
the world, and lessens the divine sovereignty. Secondly, the gravity
of a sin may be considered on the part of the sinner. Thus the sin of
one that sins knowingly is said to be graver than the sin of one that
sins through ignorance: and in this way nothing hinders heretics, if
they knowingly corrupt the faith which they have received, from
sinning more grievously than idolaters who sin through ignorance.
Furthermore other sins may be more grievous on account of greater
contempt on the part of the sinner.

Reply Obj. 1: Idolatry presupposes internal unbelief, and to this it
adds undue worship. But in a case of external idolatry without
internal unbelief, there is an additional sin of falsehood, as stated
above (A. 2).

Reply Obj. 2: Idolatry includes a grievous blasphemy, inasmuch as it
deprives God of the singleness of His dominion and denies the faith
by deeds.

Reply Obj. 3: Since it is essential to punishment that it be against
the will, a sin whereby another sin is punished needs to be more
manifest, in order that it may make the man more hateful to himself
and to others; but it need not be a more grievous sin: and in this
way the sin against nature is less grievous than the sin of idolatry.
But since it is more manifest, it is assigned as a fitting punishment
of the sin of idolatry, in order that, as by idolatry man abuses the
order of the divine honor, so by the sin against nature he may suffer
confusion from the abuse of his own nature.

Reply Obj. 4: Even as to the genus of the sin, the Manichean heresy
is more grievous than the sin of other idolaters, because it is more
derogatory to the divine honor, since they set up two gods in
opposition to one another, and hold many vain and fabulous fancies