Does it make sense to say that, "truth is relative"? Of course not, but people are often well-meaning when they say this, so be nice when you point out their error.
Where is the error? If we say that truth is relative, we are making a non-relative statement. An absolute or objective truth claim that we expect people to take seriously. This means that by saying "truth is relative" we are also implying that truth is not relative.
If that was a bit of a mindful, look at it from the negative point of view: It is an absolute truth statement to say "there is no such thing as absolute truth".
If all of that fails to make an impression, try this response: How do you know that truth is relative? What if Jesus is really YOUR truth too? What then?
If there is any absolute or objective truth (objective is usually a better word to use) then God is in the best position to know what it is.