My immediate response would probably be God’s special revelation in His authoritative and inerrant Word.
Here’s what B. B. Warfield had to say in “The Deity of Christ”:
The Scriptures give us evidence enough, then, that Christ is God. But the Scriptures are far from giving us all the evidence we have. There is, for example, the revolution which Christ has wrought in the world. If, indeed, it were asked what the most convincing proof of the deity of Christ is, perhaps the best answer would be, just Christianity. The new life He has brought into the world; the new creation which He has produced by His life and work in the world; here are at least His most palpable credentials.
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