As a Seventh Day Adventist that I am, the 7 has a special meaning for me. Seven is a godly number indeed. God created everything in seven days with the power of his Word. He could have created everything in just one day, or two, five, ten, but he decided to do it in seven days (literal or not, its what the Bible says). Then He ended the creation in the seventh day, rested, and blessed the seventh day. How tired could the Almighty, Omnipresent, Omniscient God be, after using the power of his Word? Besides asking for obedience, there has to be a special meaning to the seven...seven days in a week, seven continents, seven colors in the rainbow, seven years of plenty and seven years of famine in Pharaoh's dream...and all the other 7's in the Bible (just wiki or google it).
Michael studied the Bible, and I am sure he got the message from there. He was doing his job; letting the bad guys know he's on the good side. Just my opinion :)