Operating or being a passenger in or on a water/
land vehicle at such a velocity or in such a precarious
way that the result in losing control of the vehicle and/or simply crashing would result in an inevitable fatality, which would lead to you meeting Jesus himself, hence the term.
For example: Going over 85/90mph on the highway would most likely be considered going Mach Jesus due to the fact that a sudden stop at that
speed would kill the passengers of the vehicle. Doing dangerous stunts at high speeds and/or going more than double in a residential, although probably not fatal for the passengers, could be considered Mach Jesus.
Going so
fast in a speed
boat that colliding with another would not only destroy the
boat but more than likely kill the passengers.