If truth is relative and there are no absolute truths, how do we know that the statement, “All truth is relative”, is true? This statement claims to be true in all circumstances—no matter what, truth is relative. In other words, this statement claims that ultimate reality consists of relative truths. So, the statement, "All truth is relative”, is claiming to be absolutely true.
However, if all truth is relative, the statement, “All truth is relative” cannot be true in all circumstances. This is a contradiction at the heart of the theory that truth is relative because in order for this theory to be true, one must assume the existence of at least one absolute truth—that all truth is relative. If we make this assumption, then we cannot claim that absolute truth does not exist.
…If reason directs one to accept the existence of absolute truth, one should follow its lead. Additionally, if absolute truth exists, it must have a source, and we should attempt to discover the source as this will shed more light on the nature of absolute truth itself. –Stephen McAndrew (Why It Doesn’t Matter What You Believe If It’s Not True [Deep River Books 2012 pp. 43,47])