A matter of life and death
Definition 1 of 1
Pro Tip 1/3Extreme Urgency
Use it to express a situation requiring immediate attention or action to avoid disaster. 
The fire in the building made evacuation a matter of life and death.
Pro Tip 2/3Metaphorical Usage
Sometimes used to exaggerate the importance of a non-critical situation for effect. 
Getting tickets to the concert is a matter of life and death for some fans.
Pro Tip 3/3Emotional Weight
It carries strong emotional implications; use it to show deep concern or to stress the seriousness of a situation. 
For the stranded climbers, rescue efforts were a matter of life and death.