A commitment to relativism means that reason is ultimately irrelevant, that there is only preference. In such a world, power trumps. The thugs will win. Only a commitment to concrete truths, to argument in the light of evidence, makes humans free. Because civilized people regard one another as reasonable and free and wish to persuade one another only in the light of evidence.
Barbarians use clubs.
In the Pope's mind, the loss of faith in reason means a loss of faith in each human being. This road is forbidden to Jews and Christians, who have always seen in women and men creatures made to be intelligent, inquiring, free, bold, and not created in vain. We are made in the image of the One who conceived us, loved us, and put us where we are for a purpose. In giving us faith and reason, this God gave each of us two wings. We will each need both of them.