Romans 11:33
Romans 11:3333O, the depth of the riches of the wisdom and knowledge of God! How unsearchable are His judgments, and untraceable His ways!
Witnesses are entirely comfortable with this verse — the Watchtower teaches that Jehovah is unsearchable and that humans cannot fully grasp his ways. What does not usually get noticed is how selectively the principle is applied. The same conversation that grants God's incomprehensibility often rejects the Trinity on the ground that it cannot be pictured or explained, which quietly makes human comprehension the test after all.
This is the verse for the objection that comes before all the others: "the Trinity doesn't make sense, so it can't be true." Notice that the objection has a hidden premise — that if a claim about God cannot be fitted into a human model, the claim is false. Surface that premise gently rather than attacking it. You are not the standard of truth; neither is the Witness; neither is any organisation. God is. And here Paul says God's judgments are unsearchable and his ways beyond tracing out. says the same. Most Witnesses will grant this readily, because it is thoroughly Watchtower-friendly language. Once granted, the philosophical objection loses its grip, and the question changes from "can I picture this?" to "what has God revealed?" — which is the conversation you actually want to have. Be careful not to overplay it. Appealing to mystery is not a licence to believe contradictions, and the Trinity is not offered as one. The claim is narrower: a doctrine can be above reason without being against it, and being unable to model the Creator is what you would expect of a creature. Compare , , .