We've been considering the logic of the OT arguments for the true God. The argument is not: Think about who the true God is - the true God is actually Yahweh. The argument is: Think about Yahweh (encounter Him, see Him at work, trust Him) - Yahweh is the true God.
The former argument assumes we know who the true God is and then gets us to re-shape our view of Yahweh around that. The latter argument invites us into relationship with the tribal deity of Israel and then makes us re-shape our views of the true God around Him.
Of course the scandal of identifying Israel's tribal deity as the true God is ratcheted up several million notches with the incarnation. It's not just that the God of Abraham is the living God, it's that the Seed of Abraham is the living God! Yahweh shows up among us as an itinerant Nazarene Rabbi. He is not just God in a concrete relation, He is God as a concrete human. Not only the God of Israel but an Israelite. Nonetheless His claim is not diminished - this Jewish man, born of Mary is the LORD of Israel.
And again His identity as the LORD is seen in His concrete work of redemption.
"When you have lifted up the Son of Man, then you will know that I AM." (John 8:28)
How is the true God known? Look to this particular, historical event. Look at this act of infinitely costly service for my people. Look to my redemption.
Yet how often in evangelism do we do things the other way around? We either assume that people know 'God' in the abstract or we actively try to prove to them some kind of 'God' in the abstract (the First Cause, the Moral Legislator, the Fine-Tuning Creator). And then we try to say to them, "Jesus is actually this abstract 'God'." To which people usually frown, cock their head and set about doing the mental gymnastics required to squish the Son of Man into this pre-fab abstract-deity mould.
How many testimonies run along the lines of, "I always knew God and then the preacher convinced me that Jesus fitted the bill of the God-I-had-always-known." When this happens both 'God' and 'Jesus' are going to get majorly distorted.
Let's instead resolve to tell people, "Whatever you thought God was like, allow the LORD of Israel, the Son of God, to recalibrate all God-thoughts."
As Lord Byron once said, "If God isn't like Jesus, He ought to be." That's exactly right - that's the logic of the bible: Jesus must shape all God-thoughts. Our 'God' must be determined entirely by what we meet in the pre-incarnate LORD and the incarnate, crucified and risen Son of Man.