[Some may be offended that I use the word "sinners" instead of "unchurched", etc. I use it deliberately because it is a biblical word. What are people in rebellion to God but sinners? If that makes me old-fashioned or politically incorrect then so be it; I'm a self-professed theological dinosaur and I'm not budging.]
Can Christians have unity? Well, there has to be a compelling reason to be unified in the first place--to speak with one voice, to share one vision and implement one mission. I am convinced unity can come only through an orthodox presentation of the gospel. If it doesn't come through the Triune God then it won't come at all.
Consider Paul's words:
Now I would remind you, brothers, of the gospel I preached to you, which you received, in which you stand, and by which you are being saved, if you hold fast to the word I preached to you--unless you believed in vain. For I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures, and that he appeared to Cephas, then to the twelve. Then he appeared to more than five hundred brothers at one time, most of whom are still alive, though some have fallen asleep. Then he appeared to James, then to all the apostles. Last of all, as to one untimely born, he appeared also to me.The gospel isn't ooey-gooey-feel-good stuff. It has content, it is the gospel. There are non-negotiables. If people don't believe them then they may be church-goers but they aren't Christians.
(1Co 15:1-8 ESV)
The gospel is our rallying cry and it is the basis for our unity. We are one of a group, The Elect. We are The Remnant. We are The Saints. All of this comes through our worship of the God-Man Jesus Christ.
We may never resolve some issues this side of heaven; Wesley and Whitefield disagreed over TULIP and neither convinced the other that he was wrong. However, in the end they could hold fellowship through their love of Jesus. It was rocky, yeah, but they could do it.
Will denominations be a thing of the past one day? I sure hope so but I recognize that such a miracle would come only from the Trinity--humanity will never achieve it alone. By an end to denominations, I do NOT mean that we sweep our theological differences under the rug. As I said in an earlier post, there is a reason why I am Church of God (Anderson). I won't insult you by expecting any less for you.
Can Christianity come to consensus one day on important but secondary matters?
Only God knows--literally.