omission. Wherefore through negligence,
ignorance of what one is bound to know, is a sin; whereas it is not
imputed as a sin to man, if he fails to know what he is unable to
know. Consequently ignorance of such like things is called
"invincible," because it cannot be overcome by study. For this reason
such like ignorance, not being voluntary, since it is not in our
power to be rid of it, is not a sin: wherefore it is evident that no
invincible ignorance is a sin. On the other hand, vincible ignorance
is a sin, if it be about matters one is bound to know; but not, if it
be about things one is not bound to know.

Reply Obj. 1: As stated above (Q. 71, A. 6, ad 1), when we say that
sin is a "word, deed or desire," we include the opposite negations,
by reason of which omissions have the character of sin; so that
negligence, in as much as ignorance is a sin, is comprised in the
above definition of sin; in so far as one omits to say what one
ought, or to do what one ought, or to desire what one ought, in order
to acquire the knowledge which we ought to have.

Reply Obj. 2: Although privation of grace is not a sin in itself, yet
by reason of negligence in preparing oneself for grace, it may have
the character of sin, even as ignorance; nevertheless even here there
is a difference, since man can acquire knowledge by his acts, whereas
grace is not acquired by acts, but by God's favor.

Reply Obj. 3: Just as in a sin of transgression, the sin consists not
only in the act of the will, but also in the act willed, which is
commanded by the will; so in a sin of omission not only the act of
the will is a sin, but also the omission, in so far as it is in some
way voluntary; and accordingly, the neglect to know, or even lack of
consideration is a sin.

Reply Obj. 4: Although when the guilt has passed away through
repentance, the ignorance remains, according as it is a privation of
knowledge, nevertheless the negligence does not remain, by reason of
which the ignorance is said to be a sin.

Reply Obj. 5: Just as in other sins of omission, man sins actually
only at the time at which the affirmative precept is binding, so is
it with the sin of ignorance. For the ignorant man sins actually
indeed, not continually, but only at the time for acquiring the
knowledge that he ought to have.
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THIRD ARTICLE [I-II, Q. 76, Art. 3]

Whether Ignorance Excuses from Sin Altogether?

Objection 1: It would seem that ignor