David ends this psalm by saying God commands the blessing of eternal life. Because this is a psalm about unity, unity must be something that we do in response to His command. When God commands something to be done, there must be some response to accomplish His word. God says in Isaiah 55:11 that He sends forth His word, and it does not come back to Him empty, unfulfilled, void. This does not mean that hocus-pocus, abracadabra, something gets done. It means a "work" begins and is accomplished, and God receives it back as a completed project. He commands and gives everything needed for the work to be done, and then someone must respond and do it, presenting it back to God as a finished work.
Unity is such a work, commanded by God. We must respond to His charge and give it back to Him as a finished work, or it will never happen! Unity is something we do in response to His command that it be. It is like many other godly works or acts that consist of God and us working together to produce them. Sanctification is one of these godly acts. God does something for us, usually far more than what we have to do or can do, but even what He does is not enough. We, then, must respond to His action and carry it through for it to be accomplished. He does not say, "You are saved!" and it is done. No. We have to respond to His command—His calling—repent, be baptized, continue in His way, overcome, grow, and endure to the end. Only through this whole process are we completely saved.
Unity works the same way. God sends His Spirit—His very nature, and power with whatever gifts we need to fulfill the process—and then we take up the burden of promoting, continuing, and finishing it. Only then will we have unity. A person can pray all he wants for God to unify the church, but if he is doing nothing to build it, it will never happen.