As a Catholic of course, I see a return to the Catholic Church as the solution.
Jesus so desired us all to be one; it was His final prayer before heading to Gethsemane (John 17). To be one, means to be one in what we believe (one Lord, one baptism, one faith cf. Eph 4:5) – and there is no way that can happen without a final authority when differing doctrinal positions are proposed. Jesus knew what would happen in the future - that interpretations of Scripture and His teachings would differ among His followers. I believe that is why He established the papacy and guaranteed His protection to oversee that it would promulgate only the truth. (By the way, our trust is not in the pope personally – it’s trust in God’s power to ensure that that Pope, and Magisterium, never put forth as doctrine something that is not true. God is perfectly capable of intervening if error is about to be put forward.

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If Christianity is presently in decline, I think one of the important reasons is the proliferation of denominations. How can so many conflicting messages claim to be a witness to truth to those who do not believe in Jesus?! In John 17:21-23 Jesus connects being one with evangelizing the world.
*“I do not pray for these only, but also for those who believe in Me through their word, that they may all be one; even as Thou, Father, art in Me and I in Thee, that they also may be in us, so that the world may believe *that Thou has sent Me. The glory which Thou has given Me I have given to them, that they may be one even as we are one, I in them and Thou in Me, that they may become perfectly one, so that the world may know that Thou has sent Me …”