Ok, now nobody throw a stone at me for using evidentialist apologetics. I admit they aren't ultimately weighty. Ok? But I still think the following are a good thoughts.
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In a section of his book "Who Moved the Stone?", Frank Morison comments on the resurrection of Christ as a historical fact and begins by talking about it as attested to by the book of Acts, but then he says:
There is a far earlier and more authoritative testimony in the letters of Paul, of Peter, and of James the Just, and in the admittedly historic network of Christian churches stretching from Jerusalem through Asia Minor to the catacombs at Rome. Only from an intensely heated center of burning zeal could this vast field of lava have been thrown out from a tiny country like Palestine to the limits of the Roman world. We cannot insist on the strict reign of causality in the physical world and deny it in the psychological. The phenomenon that here confronts us is one of the biggest dislodgments of events in the world's history, and it can be accounted for only by an initial impact of colossal drive and power.
In other words, we know that physically Christianity spread very far and very fast, therefore we ought to ponder what psychological force could drive it? He continues...
Yet the original material from which we have to derive this dynamic force consists of a habitual doubter like Thomas, a rather weak fisherman like Peter, a gentle dreamer like John, a practical tax gatherer like Matthew, a few seafaring men like Andrew and Nathanael, the inevitable women, and at most two or three others.
I do not want to minimize the character of the historic nucleus from which Christianity spread, but, seriously, does this rather heterogeneous body of simple folk, reeling under the shock of the Crucifixion, the utter degradation and death of their Leader, look like the driving force we require? Frankly it does not, and the more we think of it disintegrating under the crisis, the less can we imagine it rewelding into that molten focus that achieved those results. Yet the clear evidence of history is that it did. Something came into the lives of these very simple and ordinary people that transformed them out of all similitude to the broken and shattered party of Jesus that we have recently been studying.
I feel upon examination these are two noteworthy points.
First, how could it spread so fast? I mean, we aren't talking about the decades and centuries required for 'myth' to form about some legendary resurrection (he makes this point later in his book). That the church grew and spread rapidly in the first few years is a historical fact.
Second, how could this pathetically small and weak group of individuals account for such vast effects and rapid spread of Christianity? And especially how is this so in light of the fact that they were shaken up and devastated by the death of Christ, seemingly unprepared for it - the men fled, Peter denied the Lord, and only John and the women were there at the cross. This the gospels openly admit. They don't hide the warts, and they don't have to because they can be totally honest. So how does this work?
Answer - Christ rose from the dead, appeared to his disciples, ascended into heaven, and sent the Spirit of God in power, starting at Pentecost in Jerusalem.