This verse consists of four main ideas, stemming from the phrase "So if there is…" Verse 2 will explain the expected results of these ideas. Paul's purpose in using these phrases is to emphasize how natural the conclusion should be. Of course, there is encouragement, comfort, affection, and so forth in the Christian life. If these are obvious, then the results—unity, love, and cooperation—should be just as obvious.
The first phrase speaks of "encouragement in Christ." The assumption is that the Philippian believers had experienced much encouragement from Jesus. If they've experienced encouragement, they should be able to encourage others.
Second, Paul mentions "comfort from love." This is another way in which the Philippian Christians had been blessed. Paul mentions love throughout this letter (Philippians 1:9, 16; 2:2; 4:1).
The third phrase refers to "participation in the Spirit." The Holy Spirit had also clearly influenced the Philippians. Paul mentioned the Spirit in Philippians 1:19 and will again in Philippians 3:3.
The fourth phrase refers to "affection and sympathy." These believers had experienced both of these qualities. In Philippians 1:8, Paul already mentioned, "I yearn for you all with the affection of Christ Jesus." The word translated as "sympathy" is oiktirmoi, which can also be translated as "mercy" or "compassion." This implies a feeling of relief for the forgiveness of sins, something believers experience at salvation.