That was when we first started hearing rumblings, but it was around the mid '90's (when Christianity not only lost its official status but started to become unpopular) that things really got bad.
there are all manner of other factors at play (China, for instance, is an Atheist country and while they've got fifty-five squillion problems gun violence is not one of them unless you count the amount of it that happens at the hands of the PLA), but the loss of morality altogether, which has its roots in the loss of the God, the loss of the entire Judeo-Christian cultural core, is at the root of America's problems right now.