Hardline Protestants like me say Rome is not a Christian church. Rome left the church. That sometimes provokes the question, "When did Rome cease to be Christian?"
Although that's a natural question, it contains a dubious assumption–as if that has to be a punctiliar event. But unlike individual apostasy, the church of Rome is a collective. Many moving parts. And some parts move at different speeds than other parts. Some parts are catching up with other parts.
It's like asking when did a marriage end? Legally, you could say the marriage ended when the divorce papers were signed. But that's a formality. And divorce is the effect of preceding events. One thing had to happen before another thing could happen.
Often, a marriage dies in stages. Did divorce end the marriage, or did an affair end the marriage? Before the divorce was the affair. That was a precipitating event.
But we can go back another step. Extramarital affairs don't necessarily happen just out of the blue. Sometimes one or both spouses begin to stray psychologically before they stray physically. They lose their commitment to the marriage. They become restless. They want out. They explore escape routes. They may deliberately go places where singles hang out. They arrange "chance" encounters. Although they just happened to meet someone else, they didn't just happen to be there.
Or sometimes a spouse will intentionally say or do something so hurtful, so unforgivable, to provoke divorce. To cross a line of no return.
Some people say a marriage ends when love ends. When one or both spouses fall out of love.
Yet there are couples who soldier on through the dry seasons of marriage, and emerge with a stronger marriage at the other end of the ordeal.
A dying marriage is a chain of events. Turning points. The death of a marriage may not be irreversible at any particular stage. If there's willingness on both sides, it's usually possible to restore the relationship.
But when a marriage fails, it's usually not just one crucial event, but a cumulative series of often small incidents that chip away at the foundation until that finally washes away.
Conversely, little acts of consideration can build a marriage. The dynamic goes both ways.
Because denominations are collectives, dying denominations tend to die a slow death. An incremental process. But there's a point at which you can look back and say, absent divine intervention, that's a lost cause. Too few of the faithful remain to reverse course.