Relativism is holding the view that there's no the Truth, or a central Truth. Or there are many truths, or kinda. So, it would be correct to say that relativism is denying of the truth.
If God is God, then He is not a liar. By a liar we can understand either a total liar, or a partial liar; and a latter one might be not worse liar, than a first one. And the devil is a father of lie.
But if we cannot be sure in any truth, or we cannot say that that this truth is the truth, then how do we expect to: not confuse God with a god? How can we trust any of God's words? And, a relativist doesn't expect to hear from God anything true.
Therefore, for a relativist God is a partially liar, then, according to that God is Love or God is Who creates Good, not bad or evil, it implies that either there's no God, or that God doesn't equal to God, that means it's devil.