Newtons First Law of Motion – Everybody continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise. The first law of motion can, therefore, be simply expressed as:
If the net external force on a body is zero, its acceleration is zero. Acceleration can be non zero only if there is a net external force on the body.
We know that the net external force on the object is zero. In that case we can conclude that the acceleration of the object is zero. For example, a spaceship out in interstellar space, far from all other objects and with all its rockets turned off, has no net external force acting on it.
Its acceleration, according to the First Law, must be zero. If it is in motion, it must continue to move with a uniform velocity.