Full Idea
If there is no first cause among efficient causes, there is no ultimate or intermediate cause. That in efficient causes it is possible to go on to infinity is plainly false. So it is necessary to admit a first efficient cause, which everyone calls God.
Gist of Idea
Way 2: no effect without a cause, and this cannot go back to infinity, so there is First Cause
Source
Thomas Aquinas (Summa Theologicae [1265], Ia,Q02,Art3,Reply)
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.84
A Reaction
[compressed] It doesn't seem to follow at all that the First Cause is God. There could be a single thing like the Phoenix, with unique self-causing properties. Or a quantum fluctuation.