The point of no return
Definition 1 of 2
Pro Tip 1/3Metaphorical Use
Often used metaphorically to express a critical decision point after which there is no turning back. 
He knew that marrying her was the point of no return in his life.
Pro Tip 2/3Tipping Point
Can imply a tipping point where the situation will significantly change and you must commit. 
The campaign had reached the point of no return, they had to see it through.
Pro Tip 3/3Dramatic Situations
This phrase adds drama, often hinting at high stakes or irreversible consequences. 
This was it, the point of no return, the heist was about to start.