1 Corinthians 9:23

And this I do for the gospel's sake, that I might be partaker thereof with you (KJV)

Then: Paul takes the approach in order to have the Corinthians and others reading the letter join him in eternal salvation. Wright (p. 117) points back to verse 22 which carries forward to "for the gospel's sake." For the gospel Paul cares not for his rights or his freedoms. He will deny himself so that the message, the knowledge and presence of Christ, harvests souls for the kingdom.

Now: Some are critical of the approach. An example in today's world is found on the Answering Islam web site. Some Muslims say that "becoming all things" means adapting deceptive practices for the sake of spreading the salvation message. The on-site answer is that Paul wanted to display deep-felt empathy for those needing ministry. Not hypocritical, he needed to understand the specific group of not-yet Christians. Being prepared to answer such criticism is essential to today's global mission efforts.

Extra source: "I Corinthians 9 and the Charge of Christian Missionary Deception" on line at http://www.answering-islam.org/BibleCom/1cor9_19-23.html