It's a strange thing, truth! Truth is the one single faceted thing that must not and, indeed, it will not ever change. Jesus is the truth and that truth does not rely on human belief. Humans rely on Him if they hope to avoid Hell, Judgment, and the Abyss/Lake of Fire.I don't think acknowledging that our truth, as Christians, is not the "truth" of unbelievers is dabbling in relativism. I think its just stating the obvious.
If you believe someone's disbelief in God is their truth, you have listened to the Left for to long. What they believe and teach is Relativity and the truth about Relativity is that it is a lie and there is no truth in it as far as Spiritual Matters are concerned.