The natural theology argument for the existence of God from contingency and necessity rests on the existence of something contingent. This is remarkably easy to supply, since any telling of this argument is, itself, contingent, and supplies the necessary contingent thing. However, explaining why it is contingent sometimes confuses people, because the non-existence of the contingent thing at some point in time is most typically used.
There’s nothing intrinsically wrong with this, but it can accidentally mislead people into thinking that the causal chain that must be finite (since there cannot be an actual infinity) is a temporal chain of causation. E.g. I’m here because of my parents, who are here because of their parents, and so on back to the Big Bang, which is here because of God. This can be helpful to illustrate the concept of a causal chain, but it’s not the kind that’s actually used in the argument, since it’s not the sort referenced by “actual infinity”. What’s discussed is why the contingent thing is here, now, as in, what is giving it the power to exist this moment. It cannot be something that doesn’t exist, because things which don’t exist have no power. So it must be something that also exists right now. That thing which exists right now can either be contingent or necessary, and if contingent, it too must be dependent for its existence on something else which also exists right now. And so on; this is what must terminate in something necessary because there cannot be an actual infinity.
Something that my attention was drawn to by a commentor asking me a question in one of my videos is that one can use the existence of a thing in one part of space but not another as a demonstration of contingency. If a thing were necessary and not contingent, it would exist at every point in space, since a particular location cannot cause a necessary thing to not exist. Thus anything which is someplace but not another must be contingent. The advantage to demonstrating contingency in this fashion is that space is simultaneous, and a temporal sequence will not be suggested. It is possible, then, that a person will not be accidentally led astray into thinking of a temporal sequence of events where the argument about how an actual infinity cannot exist is less clear, since the moments of time don’t exist side-by-side. (From our perspective; all moments are present to God in His eternity, of course.)