During the 4th century, there was a Roman general by the name of Flavius Constantinus. Like all powerful generals of his time, Constantinus had imperial ambitions. Being one of the four rulers of the entire empire was not enough. He desired to be sole Emperor. As civil war (a very frequent occurrence in the Roman world) erupted, it is said this pagan general was visited in a dream by who he perceived to be a great Spirit.
That great Spirit identified Himself as Jesus of Nazareth, and instructed Constantinus to have a new standard for his troops to march under. Shaped in what they would perceive to be a Cross, the general was told "in this sign shall you conquer." He complied with the vision and prepared for battle, the troops marching under this new sign. Despite being outnumbered (some reports say that his foe had double his forces), his forces won a decisive victory. Constantinus entered Rome and became known from that day forth in the history books as the Emperor Constantine, the first Roman Emperor to convert to Christianity, and one of the greatest Caesars Rome ever had.
We may never know the truth of what happened that day. Yet this story is instructive, for it provides I believe the attitude we should have as we approach the altar at Mass when it begins. For we mark ourselves with that same sign as Mass begins.
As the priest enters the sanctuary, he goes to the foot of the altar. Before even ascending the steps, he marks himself with the sign of the cross saying the words: "In Nomine Partis, et Filii, et Spiritus Sancti. Amen." In English, we know them as "In the Name of the Father, and of the Son, and of the Holy Spirit." While Constantine may have done this with aspirations of victory in physical combat, the priest (and all faithful) mark ourselves with this sign as we enter spiritual warfare. We know that through the Holy Cross, the Devil was conquered. We remind both ourselves and him of that fact when we begin the Mass. It is during the Mass this conquering is made present to the world.
Through this sign we also indicate whom the Mass is offered to. The Mass, and all of our lives, is meant to be offered to the Holy Trinity. Each distinctive person of the Trinity plays a particular role in the Mass. By signing ourselves by their name, we should call to mind those roles. We call to mind the Father who created us and desired union with us. We call to mind the Son who redeemed us when we rejected that offer of union. Finally, we call to mind the Holy Spirit who sanctifies us through the merits of Christ's sacrifice and leads us on the path towards that union. In that pledge, our dedication to the Three in One is renewed.