As Mark said, if you don't have "some" form of charge in the battery, you can't push start a car.
If the battery is flat to the point it kinda wants to turn over but won't, you can push start, below this you can't as the ECU won't turn on, which means there's no signal going to the injectors or spark plugs to fire any fuel that may be left in the fuel line as your fuel pump won't be turned on...