Philippians 2:1-2

1 So by whatever [appeal to you there is in our mutual dwelling in Christ, by whatever] strengthening and consoling and encouraging [our relationship] in Him [affords], by whatever persuasive incentive there is in love, by whatever participation in the [Holy] Spirit [we share], and by whatever depth of affection and compassionate sympathy,

2 Fill up and complete my joy by living in harmony and being of the same mind and one in purpose, having the same love, being in full accord and of one harmonious mind and intention.

Look around your town, your city, your state, and country, just look at the world, how many church denominations do you see? How many of them are in harmony? Not many, are there? But yet most of them are preaching that we must love each other and that Jesus is our Lord. Yet, there is no harmony, very little unity, and mistrust abounds between churches and denominations. Why is this?

[Not in your own strength] for it is God Who is all the while effectually at work in you [energizing and creating in you the power and desire], both to will and to work for His good pleasure and satisfaction and delight. Philippians 2:13

It is impossible for us to come together in harmony through our own strength. We can talk about being harmonious and being one, but unless the Truth that sets us free is taught: that it is Jesus causing us to do all things, we never will be one. 

That they all may be one, [just] as You, Father, are in Me and I in You, that they also may be one in Us, so that the world may believe and be convinced that You have sent Me. I have given to them the glory and honor which You have given Me, that they may be one [even] as We are one: I in them and You in Me, in order that they may become one and perfectly united, that the world may know and [definitely] recognize that You sent Me and that You have loved them [even] as You have loved Me. John 17:21-23

For us to be one, we must believe that we are actually as glorious as Jesus. That understanding has to come to us directly from the Holy Spirit. What a glorious Truth it is too: Jesus gave us His very Own glory. He gave it to us so that we could be one and so that the world could believe and be one with us too. We can not accept or believe this Truth through our own power, it has to come from our Father. That is why we are being used to study it and teach it; faith comes from hearing and hearing by the Word of God. 

We can say that we need to live in harmony, but that preaching does not do anything. The preaching that changes things is that Jesus lives to make us one with each other and that we are glorious this day in Him. This preaching is the power that will lead us to true unity. 

For He says, In the time of favor (of an assured welcome) I have listened to and heeded your call, and I have helped you on the day of deliverance (the day of salvation). Behold, now is truly the time for a gracious welcome and acceptance [of you from God]; behold, now is the day of salvation! 2 Corinthians 6:2