My most recently revised version of the Modal Third Way (MTW) goes like this:
1. Something cannot come from nothing. (Premise)
2. If something presently exists, then there was never a past time at which nothing existed. (Implied by 1)
3. Something presently exists. (Premise)
4. Hence, there was never a past time at which nothing existed. (From 1 - 3)
5. It is either necessarily the case that some temporally contingent entity or other has always existed, or a temporally necessary entity exists. (Implied by 4)
6. Possibly, there was a past time at which nothing temporally contingent existed. (Premise)
7. Therefore, a temporally necessary entity exists. (From 5 and 6)
The temporally necessary entity N must also be eternal, since there is no time at which something temporally necessary can fail to exist. Finally, N must be very powerful if it is capable of causing something as vast as the sum total of contingent entities C.
The only way out of this argument for the skeptic is to deny (6). In their assertion, some contingent entity or other must have always existed, given the present existence of something, anything. However, this view is highly implausible. As Alexander Pruss so eloquently asked: would the non-existence of all non-unicorns imply that a unicorn exists? Surely not. Why, then, would the non-existence of every other temporally contingent entity imply that some additional temporally contingent entity exists? Once again, it wouldn't.
Moreover, there are good reasons to believe that C could have possibly failed to exist as some past time. Think of it this way. If every part of a house can fail to exist, then the house as a whole can fail to exist, as well. Yes, there are instances in which the whole is not like its parts. One popular expression is that just because every part of a mountain is small, that doesn't mean the mountain as a whole is small.
The problem with the charge of a composition fallacy is that there are many instances in which the whole is like its parts. If every part of a mountain is made of rock, then the mountain as a whole must be made of rock. So, which category do contingent entities fall into? In addition to the argument advanced above, there is simply no reason to think of C as necessary, especially because each of its parts is contingent. Moreover, there is no apparent contradiction in postulating C's possible non-existence. Nothing contingent has to exist.
If these are the best objections skeptics can come up with against the MTW, then I think we're safe to say that the MTW is a knockdown and bulletproof argument for the existence of a necessary, eternal and very powerful entity. The funny thing is atheists shouldn't be so reluctant to accept this conclusion. After all, many of them hold to the temporal necessity of matter and energy. I propose that theists and atheists alike ought to accept the MTW as a rationally compelling argument. The only remaining disagreement is over whether N has any additional properties that would further bridge the gap between N and God. I think there are, but I'll save that for another post.