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.