As I’ve noted many times (e.g. here), when a thinker like Aquinas describes God as the First Cause, what is meant is not merely “first” in a temporal sense, and not “first” in the sense of the cause that happens to come before the second, third, fourth, fifth, etc. causes, but rather “first” in the sense of having absolutely primal and underived causal power, of being that from which all other causes derive their efficacy. Second causes are, accordingly, “second” not in the sense of coming later in time or merely happening to come next in a sequence, but rather in the sense of having causal power only in a secondary or derivative way. They are like the moon, which gives light only insofar as it receives it from the sun.
Concurrentism alone, the Thomist holds, can adequately account for both the natural world’s reality and its utter dependence on God. Occasionalism threatens to collapse into pantheism insofar as if it is really God who is doing everything that creaturely things seem to be doing, it is hard to see how they are in any interesting way distinct from him. (Consider that a mark of a thing’s having a substantial form rather than an accidental form -- and thus of its being a true substance, with an independent existence, rather than being a mere modification of something else -- is having its own irreducible causal powers.) Mere conservationism, on the other hand, threatens to collapse into deism, on which the world could in principle carry on just as it is even in the absence of God. (For if, as the Scholastics hold, a thing’s manner of acting reflects its manner of existing, then what can bring about effects entirely independently of God can in principle exist apart from God.)