Full Idea
Leibniz argues that if time is a thing in itself, and God is 'in' time, then God would be dependent for His existence on the existence of time.
Gist of Idea
If time were absolute that would make God's existence dependent on it
Source
report of Gottfried Leibniz (Letters to Samuel Clarke [1716]) by Adrian Bardon - Brief History of the Philosophy of Time 3 'Newton'
Book Reference
Bardon,Adrian: 'Brief History of the Philosophy of Time' [OUP 2013], p.57
A Reaction
Hence Leibniz says time is merely relations between events. Not sure what he thinks an event is. What is God made of? Is there some divine matter upon which God's existence must depend?