Ignorance and weakness are factors to be considered in our guilt, and
detract from the malice of our sins. Hence two kinds of sin, mortal and
venial. These mark the extremes of offense. One severs all relation of
friendship, the other chills the existing friendship. By one, we incur
God's infinite hatred, by the other, His displeasure. The penalty for
one is eternal; the other can be atoned for by suffering.

It is not possible in all cases to tell exactly what is mortal and what
venial in our offenses. There is a clean-cut distinction between the
two, but the line of demarcation is not always discernible. There are,
however, certain characteristics which enable us in the majority of
cases to distinguish one from the other.

First, the matter must be grievous in fact or in intention; that is,
there must be a serious breach of the law of God or the law of
conscience. Then, we must know perfectly well what we are doing and
give our full consent. It must therefore be a grave offense in all the
plenitude of its malice. Of course, to act without sufficient reason,
with a well-founded doubt as to the malice of the act, would be to
violate the law of conscience and would constitute a mortal sin. There
is no moral sin without the fulfilment of these conditions. All other
offenses are venial.

We cannot, of course, read the soul of anybody. If, however, we suppose
knowledge and consent, there are certain sins that are always mortal.
Such are blasphemy, luxury, heresy, etc. When these sins are
deliberate, they are always mortal offenses. Others are usually mortal,
such as a sin against justice. To steal is a sin against justice. It is
frequently a mortal sin, but it may happen that the amount taken be
slight, in which case the offense ceases to be mortal.