The Gospel; salvation.
Here look, please take your time prayerfully.
1Cor 9:23 And this I do for the gospel's sake, that I might be partaker thereof with you.
Paul is doing something for the Gospel's sake that he might be a partaker of it, the Gospel with the people he is speaking to.
Then he says
1Cor 9:24 Know ye not that they which run in a race run all, but one receiveth the prize? So run, that ye may obtain.
As we can see Paul relates this doing something for the Gospel to RUNNING in a race that you might receive The prize, partaking of the Gospel, obtaining salvation.
1Cor 9:25 And every man that striveth for the mastery is temperate in all things. Now they do it to obtain a corruptible crown; but we an incorruptible.
Paul states RUN WE MIGHT Receive, obtain the PRIZE, Partaking of the GOSPEL, the INCORRUPTIBLE CROWN.
He continues
1Cor 9:26 I therefore so run, not as uncertainly ( not aimlessly;not as one who has no fixed goal) so fight I, not as one that beateth the air:
Paul says he RUNS that he MIGHT Receive the PRIZE, obtaining the GOSPEL, the INCORRUPTIBLE CROWN. Not aimlessly is he running, i.e. not as one who has no fixed goal. But as one that may obtain the prize, the incorruptible crown, partaking of the Gospel, Salvation.
Then before going into examples in chapter 11 of what happens to those who do not run that they MIGHT Receive the PRIZE, obtain the GOSPEL, the INCORRUPTIBLE CROWN. Not aimlessly running, i.e. not as one who has no fixed goal. But as one that may obtain the prize, the incorruptible crown, partaking of the Gospel, Salvation.
He defines this running he is doing for the Gospel, this discipline that they MIGHT Receive the PRIZE, obtain the GOSPEL, the INCORRUPTIBLE CROWN.
1Cor 9:27 But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway (REPROBATE, REJECTED).
Paul says he runs, in that he keeps his body into subjection lest by any means he be a Castaway; A REPROBATE, REJECTED and unaccepted in respects to obtaining the prize of the Gospel, the incorruptible crown, life everlasting with the Father and Son for eternity.
Then he continues in chapter 11 and gives examples of what happens to those who do not run that they MIGHT Receive the PRIZE, obtain the GOSPEL, the INCORRUPTIBLE CROWN. life everlasting with the Father and Son for eternity.
In closing in respect to this topic he says.
Now all these things happened unto them for ensamples: and they are written for our admonition, upon whom the ends of the world are come. Wherefore let him that thinketh he standeth take heed lest he fall. There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it.
1Cor 10:11,12
I would be amiss if I failed to mention the Greek word for Castaway in verse 9:27 is a real interesting word. It is also translated REPROBATE and REJECTED in the KJV of the New Testament.