This animation shows the four forces acting on an airplane in flight. In the case of a uniform motion, the vector sum of the forces is zero.
The force of thrust as a double action: It counters the force of friction (drag), but it is also provokes the lift force that keeps the airplane in the air.
The movement of air at high speed, and the particular shape of the wings make it possible to generate the lift force.
If the thrust decreases, the aircraft slows down and its lift also decreases.