Was there something about the modern condition - science or technology, or anthropology perhaps - that constituted the discovery that there are no absolute but only relative truths?
Of course not - since this is logically impossible.
Relativism cannot be discovered - relativism is, in fact, a metaphysical assumption, something brought-to experience and used to interpret experience
(and relativism is an incoherent metaphysical assumption, as has been known since ancient Greek times, if not universally to common sense).
So what do people mean by relativism - to what aspect of modern experience do they refer when they make the assertion (or live by the assertion) that all 'truths' are relative?
By relativism they actually mean change.
The 'discovery' was change - and specifically it was the experience of change in oneself, and more specifically it was change in one's own morality - and even more specifically it was the experience of moral inversion: most crucially the experience of changing one's mind and accepting that what one had thought was sin is instead virtue.
To experience the novel conviction that sin is actually virtue: that is what is meant by relativism.
Thus the evil of relativism.