Newton's first law

The first law states that if an object is not acted by some forces, its velocity remains unchanged.

I want to know whether the following proof makes sense or have some logical problems.

I imagine that an object is at rest in a completely empty universe. It certainly remains at rest as there is nothing that may alters it.

However, I cannot claim that the object is really at rest. As an analogy, ancients do not know the rotation of the earth except from long lasting astronomical observations, but the earth surface is actually moving with approximately linear velocity if observed from outside the planet.
I may conclude that there exists unknowability for uniform velocity.

Therefore, the object keeping at rest in my view is equivalently saying that it keeps having constant velocity in the "god's view" who know its actual velocity.

So an object will not change its velocity when no forces act upon it.

If there exists more appropriate proof or deduction , please also tell me.