It's hard to say why this collection of simple-minded platitudes is so popular. Is it partly because people believe it is old? Actually the attribution to a document found in 1691 in Old St. Paul's Church in Baltimore cannot be correct. The church was constructed in 1692. Most documents published by the church, i.e., weekly church-bulletin inserts, bear that date. But this fact turns out to be irrelevant, for this piece was actually written by an Indiana poet named Max Ehrman, in about 1926, and registered with the Copyright Office of the Library of Congress early in 1927. Copyright was renewed after the author's death by his widow, which would extend the copyright another 30 years, to 1987. If someone else renewed the copyright again, it may still be protected under U.S. Copyright Laws.
Some web sites attribute this correctly to Max Ehrman, and some give additional information about him.