The words of our Divine Saviour reveal to us, what is the work and office of the Holy Ghost: 'He shall convince the world of sin.' Both in the old creation and in the new, both before the Incarnation or the Son of God and after His ascension into heaven, it has been, and it is, and it will be to the end of the world, th,e work and the office of the Holy Ghost to convince the world of sin; that is to say, to convince the intellect, and to illuminate the reason of man to know and to understand what sin is; and also to convict the consciences of men, one by one, of their sinfulness, and to make them, each one, conscious that they are guilty before God. This is the office of the Holy Ghost; and in all time from the beginning of the world the Holy Spirit of God has illuminated and convinced the intellect and the conscience of men to know God and themselves, and thereby to understand in some degree the nature of sin. But the fulness of that light and illumination was reserved unto the day of Pentecost, when the Holy Ghost came in person to dwell for ever in the Mystical Body of Christ.Therefore, my purpose is to begin by the most general outline of what sin is, and to lay down certain broad but simple principles which I shall have to apply hereafter in our future subjects. I therefore intend first to speak of the nature of sin, of what it is, and of certain distinctions of sin, which will be necessary for us hereafter to refer to.I. First, then, what is sin? There are many definitions of it, and one is this: it is the transgression of the law. 'Sin is the transgression of the law.' ...II. I have here to draw two distinctions in the nature of sin. There are what are called formal sins, and what are called material sins. The importance of this distinction you will see hereafter.1. Now let us first understand what is a formal sin. It means a sin committed with a full knowledge of what we do, and a full consent to do it; so that in proportion as men have light, and know the law and the Lawgiver,in that proportion the sinfulness of their disobedience is increased.2. This, then, is the first distinction of sin, into formal and material sin; now I must draw one more, and that is between original sin and actual sin....FROM the written Word or God it is clear, beyond controversy, that some sins are unto death, and some sins are not unto death. That is to say, that some sins are mortal, and some sins are not mortal.Our next subject, as I said, is mortal sin.