He won't save children in Africa... God doesn't exist.
I was giving an example of the same kind of question to prove that these things go nowhere. Don't turn this into a thread about religion because no one cares what you think about god in this thread.
The truth is that we can make only deductions, cause: 1) There is nothing Unstoppable if there is something immovable. 2) It depends everything from the shape, the mass, the velocity, the trajectory, the acceleration and the chemical composition of the object. 3) We need more information, we don't know if there is any other force to the moving object. So the possibilities are: 1) The Unstoppable object pierces the immovable object like a bullet. 2) The immovable object gets pushed out of the way of unstoppable object. 3) The unstoppable continues moving with the immovable object. 4) The immovable object stops the unstoppable.
A split second before impact, a black hole will appear and devour the Unstoppable Force and the wielders of both the Unstoppable Force and the Immovable Object. The Immovable Object will then remain floating inside the event horizon of the black hole forever because not even the black hole can move it.