First we need to set up what infinite past means. It means there are an infinite number of events have occurred before the present. My first point is all events in that situation would have to be real. Second is if such was, than each event from the present can be represented by a number, for example event 1 would be the event that just occurred before present and event 2 would be the event that just occurred before event one and as such to infinity.

All events in that past must have been the present at one point. If we designate the present as a point, then all points after that would be a finite amount. Since all events are numbered from the present, let's say that we ask "how many events that are even numbered and we would say infinity. Since that represents real events than there must be an event in the past that is infinite events away for even+odd=total of any number event. This is not possible for such an event would have been the present and as such can not be for that means the current present happened if an infinite number of events occurred and an addition synthesis to infinity from finite set is impossible.