Sacred art, especially painting, was carried out by those familiar with the Catholic creed and techniques: members of the clergy, for the most part, or lay people educated under their orders, so that, as the missionaries advanced through Mexican territory, painting with religious motifs expanded rapidly.
This expansion was due to the fact that sacred art was very useful for translating Christian dogmas and virtues to the native population in a language that was easier to understand. The images of suffering of Christ, Mary and other members of the Catholic calendar were a powerful vehicle of identification for the indigenous people, who suffered a profound and painful restructuring of their life as they knew it.
Source