The role of reason remains central through the opening of the encyclical:
Without truth, charity degenerates into sentimentality. Love becomes an empty shell, to be filled in an arbitrary way. In a culture without truth, this is the fatal risk facing love. It falls prey to contingent subjective emotions and opinions, the word love is abused and distorted, to the point where it comes to mean the opposite. Truth frees charity from the constraints of an emotionalism that deprives it of relational and social content, and of a fideism that deprives it of human and universal breathing-space . . . . In the present social and cultural context, where there is a widespread tendency to relativize truth, practising charity in truth helps people to understand that adhering to the values of Christianity is not merely useful but essential for building a good society and for true integral human development.
And yet, Benedict continues,
A Christianity of charity without truth would be more or less interchangeable with a pool of good sentiments, helpful for social cohesion, but of little relevance. In other words, there would no longer be any real place for God in the world. Without truth, charity is confined to a narrow field devoid of relations. It is excluded from the plans and processes of promoting human development of universal range, in dialogue between knowledge and praxis.
This is less clear, perhaps, than it should be. The suggestion that charity can live in any form as helpful for social cohesion but without any real place for God in the worldsurely that is exactly what the anti-Christian atheist believes and desires. And if it is helpful for social cohesion, then how is it confined to a narrow field devoid of relations? Social cohesion is a relation, isnt it?
The argument is probably best understood if we turn the paragraph upside down. Begin with the thought that human development appears these days driven entirely by scientific and technological advances (knowledge) as influencing and influenced by economic, political, and social uses of that knowledge (praxis). Charity doesnt cease to exist in such a worldbut it ceases to have any real place in human development. It becomes simply an indulgence, a sentiment, and a malleable emotion that can be turned to any number of dubious purposes. Love must be the third partner in the dialogue that is human history.