“God is self-existent. This means that he is without origin, and is Himself the ground of His being. He simply is. This is a most obvious truth. If God were not self-existent, He would not be infinite; for He would be determined and limited by His cause and dependent upon it. Divine self-existence is also involved in the cosmological argument. That argument teaches that God is the real and ultimate Cause of all things which are caused. But a being who is himself caused to be is not a cause at all, except in a derivative and secondary sense. He is an effect or link in the chain of causation. A true and ultimate cause must be uncaused, that is, self-evident.”
The Being and Attributes of God, Francis J. Hall, D.D., New York:Longmans, 1918, p. 252.
As explained in an earlier post, the First Cause argument refers to an essentially ordered causal series, not an accidentally ordered causal series. “[A]n essential series of causes is one in which the first, and every intermediate member of the series, must continue to exist in order for the causal series to continue as such. The illustration is a hand holding a stick which is pushing a stone. If the hand suddenly withers, losing its power of motion, then the stick in turn will stop moving, and so will the stone. Thus, the causal series will come to an end. The hand has to exist at the same time as the stick and the stone, in order for the causal series to continue; i.e. for the stone to continue being pushed by the stick moved by the hand, stone, stick and hand must all exist in the present.”
Thus, “here and now, everything that is in motion is part of an essentially ordered causal series … Nothing moves itself, but everything is moved by something else. We may say that human beings and animals move themselves. But when you get down to it, we are dependent on things outside us to keep us moving: Air, water, food, gravity, electromagnetism, strong and weak forces, etc. In other words, we are not first causes. We do cause some things, but we are also, ultimately and fundamentally, caused.”
What follows from this is that the stick, though in a sense it is the cause of the movement of the stone, is actually “an effect or link in the chain of causation”; as is every other non-self-existent thing. Thus, if God were not self-existent, he too would not be a cause in the truest sense but only “in a derivative and secondary sense.” Like us.