"In 321, Constantine instructed that Christians and non-Christians should be united in observing the venerable day of the sun, referring to the sun-worship that Aurelian had established as an official cult."
citation "http://en.wikipedia.org/wiki/Constantine_the_Great"
The "tangent" of apostasy is when other influences came into play when something other than what Jesus and the Apostles practiced/preached. In other words, when Sunday worship became mainstream and the smaller "backward" churches got pushed out of the limelight and political arena.
Yeah once again a Non answer. When did the Catholic Church come into existence?