I remember an old essay by Martin Gardner, where he spoke of two approaches in philosophy to “truth.”
The more conventional idea is that there is such a thing as truth, and that we can know some of it. We can approach it. Maybe we can’t know all of it, but, as an ideal, it exists. Think of it as a scrap of paper with the Real Answers written on it: it might be somewhere we’ll never find it, but it exists.
The other idea was that there isn’t such a scrap of paper at all. There is no “truth.” It doesn’t exist in any real sense. We can still accumulate knowledge, do science, and so on, but we’re only building up conceptual structures of our own creation, not encountering the “real” truth.
The former is much more widely held. Someone once said that science is an asymptotic approach to the truth: every advance gets us closer to it.
(Isaac Asimov said something similar in “The Relativity of Wrong.” We used to believe the world was flat. Then that it was a sphere. Then that it was an oblate spheroid. These days, we think it has some largish bulges that deform it from perfect oblate-sphericity. Each of these ideas is wrong, but each of these ideas is slightly less wrong than the idea preceding it.)
Meanwhile, your correspondent is engaging in a variant of the “ontological argument.” This classically holds that God is defined as “perfect,” and a “perfect” entity would have no flaws. Not existing is a flaw. Ergo, God must exist.
There might be such a thing as Absolute Truth…and yet there might very well be no God to “define” it. Absolute Truth might exist, but never be “defined.” The argument that God must exist to define it is mere linguistic masturbation. It’s almost as foolish as the argument that, since there are “laws of nature,” there must be a “lawgiver.”
He is right that there is no “Absolute Certainty” in atheism. He’s just wrong in imagining that there is Absolute Certainty in any other idea. There isn’t. His faith is based on…faith. He’s welcome to it, but it’s hubris for him to claim it’s “certain.”