There exists what is truly and absolutely right and we can discern what that is, there is a standard and the standard is what is perfectly right and good. If we were unable to know what was right and good then there would be little reason for us to exist as we could not live a good life or even know what that entails, everything dealing with morality and conscience would be a subjective absurdity.
Without this "standard" so to speak, the choice between right and wrong would be subjective. So any action from murder to theft would be acceptable in this world. Nothing would matter.
We know that such a model or standard exists because we trust that God did not create some sort of silly and simple-minded game for humans to take part in.