The priest addresses the congregation at Mass four times with the words, "The Lord be with you."  That is, Almighty God desires to be with us through the priest, and the priest expresses this in these words.  But in what ways does The Lord desire to be with us?  In his commentary on the Mass, St. Albert the Great (d. 1280) gives his considered answer:

"We say that the Lord wishes to be with us through the priest in five ways: in our conformity to his will, in his works of power, in the showing of his power, in the participation in sacramental grace, and in the conquering of our enemies.  

"Of the first of these, it is written: Not as I will, but as you will (Matthew 26, 29).  Let your will be done, not mine (Luke 22, 42).  This conformity is four-fold.  The first is conformity to the matter that is willed – when we will that which God wills.  Not what I will, but what you will (Mark 14, 36).  The second is in the form of the willing: that from the same charity as that by which God wills what he wills, we might will what we will.  May your will be done on earth as it is in heaven (Matthew 6. 10).  The third is in the end of the will, that we might will what we will for the same end as that for which God wills what he wills – that is, for his glory.  Let them see your good works and give glory to your Father, who is in heaven (Matthew V. 16).  Whether you eat or drink or whatever else you do, do all things for the glory of God (I Corinthians 10, 31).  The fourth is conformity in the efficient cause, that I may will that which God wills me to will, as when he gave a commandment, he willed that I will to obey the commandment.  Similarly, when he gave me a counsel, he willed for me to will to follow his counsel.  And when he made me a father, he willed for me to will that I care for my children, and so on.  This is the will of God: your sanctification, that you might abstain from fornication, that each of you might know to possess his own vessel in sanctification and honor – not in the passion of lust, as the pagans do, who do not know God (I Thessalonians 4, 3).  Thus, the Lord willed to be with us in our conformity with his will, for the will cannot wander in God.

"The second way in which he is with us is in his works of power.  Behold, I am with you all days, even to the end of the world (Matthew 27, 20).  You have worked all our works for us (Isaiah 26, 12).

"The third way in which he is with us is in the showing of his divinity and power.  In Judges 6, 12-13, the angel first says to Gideon: The Lord be with you, O most valiant of men!  Gideon responded: If God is with us, why have all these things come upon us?  The angel said: Go in your strength, and you shall deliver Israel from the hand of Midian.  And so it came to pass.

"The fourth way in which the Lord is with us is in our participation in sacramental grace.  The Lord is with you.  Blessed are you among women (Luke 1, 28).  This is said in reference to the divine participation by which Mary had God with her in her heart.  

"The fifth way in which he is with us is in battle.  O Judah and Jerusalem!  Do not fear, do not be frightened!  Tomorrow you shall go out against them, and the Lord shall be with you (2 Chronicles 20, 17)."