Must a Christian disagree with Bernard Williams and say the concept of truth does have a history?

You're getting mixed up by using words with different senses as if they had only one sense.

Truth doesn't have "a history" in that it is timeless - what is really True now was True in the past and will be True in the future.

God likewise, in his being, is timeless and immutable. God is the same yesterday, today, and tomorrow.

But God's interactions in this universe, in human history, occurred of course at particular times. Jesus was born, he lived, he died, and he was raised back to life, at one time in history and not at another time in history. It helps to think of the Gospel as a covenant: a promise and agreement made between two parties at a particular point in time. God's love, mercy, grace, justice, and wrath at sin are all timeless truths. The Gospel is a timeless truth in that no one will ever be born too late to be included in it. But it had a beginning at a particular point in time, before which God's mercy and grace were revealed and experienced through different means.

Short answer

Once we understand precisely what Scriptures mean by "God is truth", a Christian CAN AGREE with Bernard Williams that "concept of truth has no history", despite progressive revelation. Truths about God are about His essence which is necessarily outside time and absolute. Otherwise, we Christians will not have the bedrock certainty of the foundation of our salvation: that God will remain faithful to us to the end of the world. 1) The manifestations of God's faithful demonstrations to us can change in history as His actions were cumulative, and 2) our understanding of Him can change as evident in the historicity of theologies, but His character (exhibited primarily through His unfailing love and faithfulness) does not.

You wrote:

Now, let us say Christians literally believe God is Truth. It is the very same; the two concepts must mean the same thing, for a Christian.

(from comment) But the meaning of "Knowing God" does literally seem to shift in history--before Christ's life it does not mean the same thing as after him--it's literally true after his Resurrection only if you "Know God" through Christ.

Progressive revelation means that truths about God are cumulative; previous truths were not replaced, but refined. Before Jesus, God's righteousness, holiness, power (as beyond all other gods), and faithfulness to his chosen people were demonstrated in the OT. After Jesus, those attributes remained true but made clearer through additional demonstration by God through Jesus's voluntary sacrifice on the cross as the blood for new covenant (Luke 22:20, 1 Cor 11:25).

He has always been the same, but the history of his actions in time is very much a history--he has not acted the same in all places and times.

Yes, the actions themselves were in history, but be careful what you mean by "not acted the same". Different action, but same character. When it comes to actions, truth about God is about God's character.

Before the life of Christ, men and women could not achieve salvation through Christ.

This is wrong, since Christ's redemption works backwards as well as forward (even to what is future to us now). Several Christian traditions teach that in between death on the cross and resurrection, Jesus went to hades to liberate the righteous OT people (such as Abraham) and resurrected them.

Longer answer (includes what Bernard Williams mean)

This is a great question and while I don't have time to write a fully polished answer, here's an outline:

1. First, as to the meaning of "God is truth". The gotquestions.org website article "What does it mean that God is truth?" makes clear that the statement "God is truth" speaks of God's morality, meaning that God does not lie, and presented in the Scriptures as righteous and holy.

2. What's the implication? Truth here is relational. In the Psalms God is often praised as can be relied on His unfailing love and faithfulness (sample verses here). When God makes a promise (as to Abraham), God will be proven faithful in the history of His dealings with His people Israel. Another example is when God fulfilled His promise of the new covenant to send Jesus about 400 years later.

3. Did people's understanding of God change since Abraham to Jesus? Absolutely. Did their way of knowing God change? Absolutely. But did God's faithfulness and unfailing love change? No, similar to how we as parents love our kids the same but from the kid's point of view our love's manifestation changed from when they were babies through teenagers through adulthood.

4. Turning to Bernard Williams's "the concept of truth has no history", it's important to see the quote in the context of where he said it: in his 2002 book Truth and Truthfulness: An Essay in Genealogy. At the end of chapter 3 of his book (page 61) and in the Endnote ("The Vocabulary of Truth: An Example, pages 271-277) we get clarification that what he means is that:

   • although the way cultures express truth differ (in the Endnote he delves into various nuances of the vocabulary from ancient Greek to modern German, French, etc.),
   • although there exist a history of theories of truth,
   • although there are different conceptions of Accuracy and Sincerity,

   everyone's concept of truth is the same. In other words, he's defending the necessity of the objectivity of truth assumed by common sense, in contrast to Nietzsche's denial.

5. THEREFORE, although there IS a history of our understanding of God (which became clearer over time and culminated in Jesus), I argue that this history is equivalent to what Bernard Williams himself acknowledges as changing vocabularies, theories, and conceptions of truthfulness (i.e. accuracy and sincerity).

6. In other words, just as Bernard Williams defends the objectivity of truth ("concept of truth has no history") despite the existence of the history ("genealogy") of "truthfulness", Christians similarly defend the unchanging "God is truth" (properly understood as unfailing love and faithfulness) despite the history of Jewish + Christian theology in the past 4,000 years.

Further reading

1. Bernard Williams "advertising" his own book in London Review of Books
2. Notre Dame Philosophical Reviews book review.
3. Philosophers' Imprint 2018 book review: Williams's Pragmatic Genealogy and Self-Effacing Functionality
4. 2020 Master's thesis The Concept of Truth in the Gospel of John by Bastian Ogon.

God has revealed himself. Progressively.

With each revelation, more can be known and understood.

The philosophical thesis you quote (and this may make the question off-topic on this site) falls down completely when one considers that God can only be known (by humans on earth) through the revelation that God gives of himself.

Until he gives it, at a point of time, that truth is not known and cannot be known.

Otherwise He who is truth cannot be known at all.

Therefore, truth does have a history.

That is to say, the body of truth, as made known by God in his word (and documented in holy writings) is a progressive revelation and has a history of such documentation.

By fully studying that documented revelation, you would then realise that the statement :

... before the life of Christ, men and women could not achieve salvation through Christ . . .

. . . . is incorrect according to the progressive revelation of Deity recorded in holy scripture.

(You could ask more about that in a further, dedicated, question.)