He who goes beyond the bounds of true moral liberty, breaks the law of God and sins.  He thereby refuses to God the obedience which to Him is due.  Disobedience involves contempt of authority and of him who commands.  Sin is therefore an offense against God, and that offense is proportionate to the dignity of the person offended.

The sinner, by his act of disobedience, not only sets at naught the will of his Maker, but by the same act, in a greater or lesser degree, turns away from his appointed destiny; and in this he is imitated by nothing else in creation.  Every other created thing obeys.  The heavens follow their designated course.  Beasts and birds and fish are intent upon one thing, and that is to work out the divine plan.  Man alone sows disorder and confusion therein.  He shows irreverence for God’s presence and contempt for His friendship; ingratitude for His goodness and supreme indifference for the penalty that follows his sin as surely as the shadow follows its object.  So that, taken all in all, such a creature might fitly be said to be one part criminal and two parts fool.  Folly and sin are synonymous in Holy Writ.  “The fool saith in his heart there is no God.”

Sin is essentially an offense.  But there is a difference of degree between a slight and an outrage.  There are direct offenses against God, such as the refusal to believe in Him or unbelief; to hope in Him, or despair, etc.  Indirect offenses attain Him through the neighbor or ourselves.

All duties to neighbor or self are not equally imperious and to fail in them all is not equally evil.  Then again, not all sins are committed through pure malice, that is, with complete knowledge and full consent.  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.