What does 1 Corinthians 9:9 mean?

Paul is building his case that, as an apostle of Jesus, he has the right to demand financial support from those he serves with the gospel (1 Corinthians 9:1–7). This would include the Corinthians. They know well that he has not claimed this right from them. This, of course, is the real point of Paul's argument here: that it is good for a believer to sacrifice their "rights" for the good of other believers (1 Corinthians 8:7–13).

Here, Paul turns to the Old Testament law of Moses to show legal precedent for this right. He quotes Deuteronomy 25:4 to show the restriction against muzzling an ox when it is treading out the grain. Farmers were not allowed to prevent the ox from feeding on the grain it was helping to process. Since the ox's labor was what produced that grain in the first place, it was only fair—and sensible—for the ox to be supported by the fruits of that labor. This fits Paul's principle: it is most natural for apostles, workers, and even oxen to be paid from the work they are doing.

This verse also adds a question about whether God is concerned about oxen. The most obvious answer is that, yes, God is concerned enough about oxen that he sees to their well-being. As shown in the following verse, Paul's implication is yet another question: how much more must God be concerned about the wellbeing of apostles?

Bible scholars suggest several alternate understandings about Paul's question here, but his main point is clear: Those who do the work have the right to be supported by the work, including apostles. In this specific context, Paul uses this to show that he is living out the teaching he gave in the prior passage.