In the poem "<em>The Second Coming</em>" written by William Yeats, the author explores Anarchy and chaotic scenarios where things fall apart and order can only be reestablished by the return of <em>Jesus Christ</em> on earth.
When the author uses the phrase "<em>The ceremony of innocence is drowned</em>" he refers to a set of circumstances of violence, brutality and horror caused by what he calls "<em>a loose blood dimmed tide</em>" which suffocates purity and freshness, describing a cataclysm; worthy of resemblance to biblical passages of events involving destruction on a catastrophic scale such as Noah’s flood, or the total and final destruction of the world, described in the book of Revelation as "<em>The Apocalypse</em>".
The poem seems to be the author's emotional release after the presumable trauma of <em>World War I</em> around that time in Europe, since it was written in 1919.
It's an evident appeal to a Christian concept in seek of spiritual refreshment as a cathartic method to purge his likely repressed emotions in the post-war environment.