City Of God: "the unabridged version".
The yoke is a symbol of discipline because God requires all men to plow the gound of their lives and produce virtue.
There are people who will say that running the race as quote by Saint Paul refers to the reward for the saved and is not refering to being saved. Such people insist that when people call upon Jesus as Lord and Savour they are forever saved and can't lose their salvation. No where in Scripture does it say you can't lose your salvation.