An airplane has an engine that pushes its flight. What force pushes a glider to fly?
Airplanes do not fly because of their engines. Airplanes and gliders both fly because their wings turn thrust into lift and drag.

That simplistic answer, of course, just begs the question "how can you generate thrust without an engine?"

A glider is constantly trading altitude (potential energy) for airspeed (kinetic energy). Energy over time equals force, which in this case we call thrust. Airplanes do the same thing when descending. Both can also trade energy in the opposite direction, but only briefly until the wings stall.

The big difference is that an airplane can also turn fuel (chemical energy) into thrust, which allows sustained level or climbing flight. Gliders don't have that option.