Why was it so important for a knight to take a vow of loyalty to his lord? The knight needed to be sure that the lord would protect him in exchange for his service as a skilled warrior.
The knight wanted to be sure that his son would inherit his position when either the knight or the lord died.
Because the lord was a Christian, it was essential for his knight to vow to become a Christian as well.
Since a knight was a skilled warrior, the lord could be in great danger if the knight betrayed him.