You're confused because you think "at rest" and "in motion" are different things, but they're not. They're the same.

Imagine two space ships, Firefly and Enterprise, in deep space set on a collision course. The captain of the Enterprise radios the captain of the Firefly saying, "Firefly, you're moving towards us at 1000m/s. Please engage reverse thrusters to match our velocity or alter your course." The captain of the Firefly responds, "No, Enterprise. YOU'RE moving towards US at 1000m/s. You engage YOUR reverse thrusters or alter YOUR course."

Who is right? Both, and neither. Both are correct from their own inertial reference frame, and both are incorrect from the other's inertial reference frame.