Explanation of the Prayers and Ceremonies of Holy Mass, Dom Prosper Guéranger - Judica, PS. XLII.

Having made the sign of the Cross, the Priest says the Antiphon: Introibo ad altare Dei, as an introduction to the 42nd Psalm.  This Antiphon is always said, both before and after the Psalm, which he at once begins: Judica me Deus.  He says the whole of it, alternately with the Ministers.  This Psalm was selected on account of the verse Introibo ad altare Dei: I will go unto the altar of God.  It is most appropriate as a beginning to the Holy Sacrifice.  We may remark here, that the Church always selects the Psalms she uses, because of some special verse which is appropriate to what she does, or to what she wishes to express.  The Psalm, of which we are now speaking, was not in the more ancient Missals: its usage was established by Pope Pius the Fifth, in 1568.  When we hear the Priest saying this Psalm, we understand to whom it refers:- it refers to our Lord, and it is in his name, that the Priest recites it.  We are told this by the very first verse: Ab homine iniquo et doloso erue me: deliver me from the unjust and deceitful man.

The verse here used as an Antiphon, shows us, that David was still young when he composed this Psalm; for, after saying, that he is going to the Altar of God, he says: Ad Deum, qui laetificat juventutem meam: To God, who giveth joy to my youth.  He expresses astonishment at his soul being sad; and, at once, cheers himself, by rousing his hope in God; hence, his song is full of gladness.  It is on account of the joy which is the characteristic of this Psalm, that holy Church would have it be omitted in Masses for the Dead, in which we are about to pray for the repose of a soul, whose departure from this life leaves us in uncertainty and grief.  It is omitted, also, during Passiontide, in which season, the Church is all absorbed in the sufferings of her divine Spouse; and these preclude all joy.

This 42nd Psalm is an appropriate introduction to the Mass, inasmuch as it in our Lord whom it will bring among us.  Who is He that is to be sent to the Gentiles, but He that is Light and Truth?  David foresaw all this; and, therefore, he uttered the prayer: Emitte lucem tuam et veritatem tuam.  We take his prayer and make it ours; and we say to our heavenly Father: send forth Him, who is thy Light and thy Truth!

The Psalm having been terminated by the Gloria Patri, and the Antiphon repeated, the Priest asks for God's assistance, saying Adjutorium nostrum in nomine Domini: Our help is in the name of the Lord: To which the Ministers reply: Qui fecit coelum et terram: Who hath made heaven and earth.  In the Psalm just recited, the Priest expressed his ardent desire to possess our Lord, who is Light and Truth: but the very thought, that he, - a sinful creature, - is about to meet this Lord of his, makes him feel the need he has of help.  True, - God has willed this meeting; he has even vouchsafed to prescribe it as one of our duties: and yet, notwithstanding, man is continually made to feel his unworthiness and nothingness.  Before going further in the holy Sacrifice, he is determined to humble himself, and confess that he is a sinner.  He encourages himself to this, by making the sign of the Cross, and imploring God's aid.  He then begins the avowal of his sins.