You should not gauge the validity of any belief on how mainstream it is -- Christian Universalism or otherwise. Truth is never popular. The crowd is often wrong. So I would caution you against this line of reasoning. It’s fallacious. I do, however, think you bring up an important point about Christ leading the Church.

It must be remembered that Christian Universalism is at its root making a claim about the final event in history, the reconciliation of all mankind. We are peering over the edge of the horizon. Not everyone can do that. Some have their eyes firmly fixed on the here and now. And while we Christian Universalists read the Bible in light of that final victory, derive comfort and joy from that victory, and see God’s love clearer through that victory, the knowledge of it is not strictly necessary to following Christ. Loving God with all your heart and your neighbor as yourself is not dependent on the knowledge of that future event. So it would be a mistake to dismiss Christian Universalism because few hold to it. Christ may not reveal the same truths to all people at the same time. 

Think about the Christians you know and consider how each of them is at a different stage in their spiritual development. Some obey God out of fear. In their infancy, they only see punishment and hell. Others are further along and see God’s grace. They obey for the precious gifts God promises: heaven, streets of gold, mansions in the sky. And yet others, who are even further along, see God as Father. They obey not out of fear or selfish interest, but because they love their Father. Their reward is Him. Each captures one angle of the truth, but the highest and noblest view is the last. I would ask you to view Christian Universalism in a similar way, as a higher rung on the ladder, a more mature view. Not all are there yet, but all will be there someday.