For walking into something. It is a prayer of protection from Ireland — Christ with me, Christ before me, Christ behind me — and a lorica, which was a breastplate, which tells you what it was for.
It surrounds you. That is the literal shape of the famous part — before, behind, beneath, above, right, left, lying down, sitting, rising — and it does not stop at your own body but goes on to everyone who will see or hear or think of you today. It is a prayer for a person who expects the day to be dangerous and intends to go anyway.
A lorica was a Roman soldier's breastplate. The Irish monks borrowed the word for this kind of prayer — a thing you put on in the morning.
The story says Patrick composed it on the road to Tara in 433, to pass unseen by the men sent to kill him, and that he and his monks appeared to the ambush as a herd of deer. The text survives in a manuscript of the eleventh century that tells that story alongside it.
The language settles the question the story cannot. The Irish it is written in is eighth-century Irish, three hundred years after Patrick died, and no scholar since the language was dated has thought it his. It is a prayer written in his name by someone who knew what kind of prayer he would have said. That is not nothing; it is how most of the prayers on this site were made.
It stayed Irish for a thousand years and then went everywhere at once. Cecil Frances Alexander turned it into a hymn in 1889 — I bind unto myself today — for St Patrick's Day, and it is sung across the English-speaking world, usually in a setting by Charles Villiers Stanford. The Christ with me section is said on its own by people who have never heard the rest, and it is on the pocket page's shelf in spirit if not in fact.