Can God make a rock so big (immovable object) that even He (an unstoppable force) can't lift it?
This question really reduces to: Can God be both unstoppable and not unstoppable at the same time? The answer is no. "A" cannot be both "A" and "~A" at the same time. To affirm that "A" can be "A" and "~A" at the same time would be a violation of the law of non contradiction.
God is all powerful only. God is not "not all powerful." The denial of the negative, "God is not 'not all powerful'" simply serves to reinforce the positive "God is all powerful," and in no way detracts from or diminishes God's attribute of "all powerfulness."
* If God can't move the rock, then God was never really unstoppable.
* If the rock moves, then it wasn't ever really immovable.
In short, it is rationally absurd to assert at the same time that:
1. God is an unstoppable force.
2. God is not an unstoppable force.
He can only be one or the other, unless God is a contradiction, in which case, God is nonsense, and nothing can be known or said about Him, including whether or not He exists. Thus, the avoidance of logical contradiction (and hence the avoidance of nonsense) requires us to say something negative about God.
Either God is not (1) unstoppable, or God is not (2) not unstoppable.
The Bible affirms that God is not (2) not unstoppable.