Sometimes people say, “Truth is relative, so if I say there is a God, then there is, and if you say there isn’t a God, then there isn't.” How would you reply? I think this statement is similar to "I have my beliefs and you have yours, so let's agree to disagree."
This is an easy one, because the truth about matters of fact isn’t relative. Two people may have different beliefs about whether God exists, but if He exists, then He exists for both of them, and if He doesn’t, then He doesn’t for both of them. Suppose your friend said, “Truth is relative, so if you say the coffee has been poisoned with arsenic, then it has been, and if I say it hasn’t, then it hasn’t.” That would be foolish, because if the coffee has been poisoned, then saying that it isn’t won’t make it safe to drink. After all, you and your friend don’t live in different universes!
My favorite line concerning the notion that truth is relative comes from G.K. Chesterton, who commented on the absurdity of saying “This is the south aspect of Sea-View Cottage. Sea-View Cottage, of course, does not exist.” It’s true that from the south, one sees the north side, and from the north, one sees the south side. But there is a house, and it is the same for everyone.
You remark that the statement “Truth is relative” is similar to the statement “Let's agree to disagree." I would say they are quite different. Disagreement between two persons about a state of affairs is possible only because they agree that there is a state of affairs, and they have different opinions about what it is. For if Fred and Mary agree to disagree about whether the water is boiling, at least they agree that if it is, then the claim that it isn’t is incorrect. By contrast, someone who says truth is relative won’t even admit that.
So two people who say “Let’s agree to disagree” aren’t denying that there is a real state of affairs. They are merely saying that for whatever reason – which may be good or bad -- they don’t want to talk about it.