There seems to be the following contradiction (or, at least, puzzle) in saying that a necessary being explains the contingent world. Consider.
It is normally assumed that a necessary being (NB) can explain the contingent world (CW) by some causal action taking place in the NB. However, this seems viciously circular. For the causal action that gives rise to the CW cannot be in the CW, since it accounts for it. But if the action is in the NB, then it is an item either itself necessary, or contingent. If contingent, then we must account for it by some further contingency, etc, until we get to some causal action that is necessary. But then, this action must necessarily be in the NB. In which case, the action which gives rise to the CW is necessary. For given the NB, it’s causal action will necessarily be present in it, and the CW will necessarily exist.
Thus, if a NB causally accounts for a CW, the CW is likewise necessary.
Another way to put the problem is like this. What explains the CW? Such a cause is either contingent, or necessary. If necessary, then, it must necessarily be what it is and cannot be otherwise. For any contingency in the cause must be explained by some prior necessity. But then, when we reach this necessary cause, its action must be necessary: it must necessarily do what it does. But what it does is cause the CW. But then, the CW is no longer contingent, but necessary.
Now, this appears to only be a contradiction on Thomistic metaphysics. For if God is temporal or everlasting it seems a NB could cause a CW by a spontaneous act of freedom. This may be mysterious or even incomprehensible – but it also may avoid the contradiction above.