What event marked the beginning of the Reformation?

It's debatable. Most say it was Luther's nailing of his theses to the door of the Church at Wittenberg.

Yes, debatable, perhaps the political climate, If the princes of Germany had not went to the protection of Luther and used that event to make a political break, I am not sure that first Luther would have lived, but second that any movement religiously would have started but again that is just a opinion

I'll think it was the mailing of the these to the Church door, but it could have been what had influenced Luther to do so.
Also the fact that the royalty was looking for an excuse to make a break with The Church and the fact that Islam was waging war on parts of Europe at that time causing additional unrest.
Peace and ,kindness,
Fred

Yes, I think it is debatable as well primarily because John Wycliffe predated Martin Luther by several years but his actual confrontation with the established Church authority is noted by some historians as the "Morning Star" of the Reformation.

Whether Luther was influenced by Wycliffe, I do not know but if we are strictly talking about whose defiance against the papal authority was more influential, perhaps that is the point of debate.

jakester,
Yes it is a point of debate.
Fred