After the call to unity in 1:27-30, Paul tells us the logical and practical ways we can achieve Christian unity. And all of these have to do with our mindset.
Philippians 1:27-30
Paul states in 1:24-26 that he anticipates his release and that he will come to minister again in Philippi. But given some uncertainty as to when he will arrive, he focuses their attention on one purpose: "Only [this is the singular focus] conduct yourselves in a manner worthy of the gospel of Christ" (1:27a). He calls them to live worthy of the gospel. Now, the Philippians weren't newbies in the Christian faith. They engaged in the defense and confirmation of the gospel (1:7). They supported the gospel ministry (1:5). And they had done this for 10+ years! Do they really need such an elementary instruction? They did, because they lacked a crucial area of this worthy conduct: unity.
Philippians 1:18b-26
In the previous section, we saw Paul rejoicing in the progress of the gospel (1:18a) even despite malicious intent by some. Immediately after this, he assures his audience that his joy will continue into the future. He says, "Yes, and I will rejoice" (1:18b). And with this statement, Paul shifts his focus away from what has transpired to what he expects will occur in the future.
Philippians 1:12-18a
After his prayer, Paul gives an update on his circumstances. His audience already knew of his imprisonment and even sent to minister to him. But he now wanted them to know something unexpected about his incarceration: "that my circumstances have turned out for the greater progress of the gospel" (1:12). How did Paul's jail time lead to an advancement of the gospel? Paul explains in the remainder of this passage.