Paul's letters, written just fifteen years to twenty-five years after the death of Jesus's, provide an outline of all the events of Jesus's life found in the gospels--his miracles, claims, crucifixion, and resurrection. This means that the Biblical accounts of Jesus's life were circulating within the lifetime of hundreds of who had been present at the events of his ministry.The Reason for God p. 101.
I guess that Keller gives himself some wiggle room by only claiming that Paul "provides an outline," rather than asserting that Paul corroborates that Jesus was a miracle worker or that he made "claims." Without having Paul on board, the apologist has to deal with the possibility that the Biblical accounts weren't circulating earlier than forty to sixty years after the events. That leaves aside the question of how long it took for the gospels to get into general circulation after they were composes. If we were to go by unambiguous external references to the gospels, we would find it hard to establish that the stories of Jesus as a miracle working rabbi were circulating much earlier than a century after the events.