Like athletes, boxers too were known for their disciplined lifestyle. One of the training habits of Melankomas of Caria (a renowned boxer in Paul’s time) was to walk with his arms stretched out high in the air for two days!
Using two strong verbs, Paul compares his own lifestyle to that of a boxer. Discipline
literally means, to strike under the eye.
A boxer subjects his body to the pain of fighting, in the hope of ultimately winning the match. Likewise, Paul subjects his body to pain for the sake of the gospel.
The second word, keep…under control,
literally means, to enslave.
Paul has made his body a slave
to a higher goal: the goal of preaching the gospel (compare 1 Corinthians 9:19).
In this regard we may again think of Paul’s hard manual labour (1 Corinthians 9:6), but also of the many forms of physical suffering that he had to endure on his missionary journeys (see 2 Corinthians 11:23–27).
Paul’s own faith is proved by his self-exertion for the sake of the gospel (compare 1 Corinthians 9:23). Without self-exertion, he (and by implication, the Corinthians) runs the danger of being disqualified
from winning the prize of eternal life.
In the following verses (1 Corinthians 10:1–11), Paul will reinforce this warning, pointing to the example of the Israelites in the desert. Through their disobedience, they were disqualified from entering the Promised Land.
27 But I discipline my body and keep it under control, lest after preaching to others I myself should be disqualified.