Final answer:
The sub theme being emphasized when the author of Sir Gawain and the Green Knight mentions that the knights attend mass and Sir Gawain prays is religious devotion.
Step-by-step explanation:
The sub theme being emphasized when the author of Sir Gawain and the Green Knight mentions that the knights attend mass and Sir Gawain prays is religious devotion.
This is evident from the mention of the knights attending mass and Sir Gawain praying, which highlights their commitment to their Christian faith.
The concept of religious devotion is significant in the medieval period, where chivalry and knightly behavior were expected to align with Christian values.