The individual, the institutions, and the community—the three protagonists in the Divine Plan—are being shaped under the direct influence of His Revelation, and a new conception of each, appropriate for a humanity that has come of age, is emerging.
This use of the indefinite article is grammatical, despite the fact that it is used with humanity, which is an abstract noncount noun.
What's meant in the text is a humanity of a kind that will be able to harmoniously integrate the yet-unshaped but emerging concepts.
Note how the word humanity is post-modified with the phrase "that has come of age". What's meant here is not just humanity at any moment in history. The indefinite article is used to stress that this "humanity" stands apart from "humanity in general".
Maybe as things stand today, humanity will refuse these concepts offered by religion. But, according to the text, "a humanity that has come of age" would find these concepts appropriate. Maybe humanity will only reach such condition in the distant future.
Quirk et al. in their book on grammar call this "the partitive effect of the indefinite article". They use the following example:
- She played the oboe with sensitivity.
- She played the oboe with a sensitivity that delighted the critics.
In sentence 2, it's not just "sensitivity in general", but a particular sensitivity, as evidenced by the postmodifying clause "that delighted the critics".
We can make a similar comparison for humanity:
- Christ brought His Revelation to humanity.
- Only a humanity that has come of age will fully adhere to His Commandments.
- The story of Christ would've ended better if only he brought His Revelation to a humanity that has come of age.
Reference: Quirk et al., A Comprehensive Grammar of the English Language, Chapter 5.59.