Christian union is built around the fact that we are all partakers with one another in the grace of God. Unity, then, will vary from person to person. With some, it will be tenuous because the relationships and experiences together are just not that strong. On the one end, we have casual acquaintances. We know these acquaintances are part of the church of God, and thus we share a Spirit with them, as well as a hope, a dream, a goal. We are on a pilgrimage with them to God's Kingdom. Because they are in our minds, we have a tenuous union with them.
On the other extreme is the union that we have with our mate, who likewise shares with us the same Spirit, the same hopes and dreams. However, with our spouse, we share a great deal more intimacy and far more experience. Our union with him or her is far deeper. Spiritually, this also applies to our unity with the Father in heaven.
This epistle was written while Paul languished in prison. His fond memories of his experiences with the Philippians made him feel confident, as if he were not alone, as if they were with him in his chains, giving him encouragement in his desperate situation. It is as if he is saying, "Because of our unity, I can feel your support."