Arguments for God's Existence from Contingency

[Neo-Scholastic]

Second, there could always have been one more event in the way distant past and we would never have arrived at today. 

FYI, you need to distinguish between accidental and essential series. In theory, an accidental causal series could have an infinite temporal past, depending on one's theory of time. Only an infinite chain of essential causes is impossible since it requires at least one necessary member. Also, you need to be careful not to adopt Hume's redefinition of efficient causes as sequential events. Prior to Hume an efficient was defined as any agent necessarily present during an event. For example, the baseball is the efficient cause of the broken window; however, the ball hitting the window and the window breaking are simultaneous parts of a single event.

(Don't expect the atheists to understand, or care to make, any of these subtle distinctions, but you should be aware that they are, IMHO, necessary distinctions for the demonstration to work)