Christian unity frequently seems to be a reality in theory only. There are so many different opinion groups within the Christian world, so many denominations, and so many subgroups even within those denominations.
The primary reason for disunity in general, including in the Christian church, is that we humans have been damaged by the fall and now value power over truth. The result is inevitable – disunity. But a lot of disunity doesn’t come from active fighting, but from the way different people groups and historical groups have grappled differently with how the gospel applies to their particular circumstances. So while this might appear to be disunity – and in some sense of course it is – the way these different groups believe in the heart of the gospel means that they are experiencing a profound degree of unity. But on another level, sometimes Christian groups (for instance, some American denominations supported slavery in the pre-Civil War South) who confess basic Christian doctrine but whose practice undermines those beliefs, must be rejected in that bad practice. Because of this, sinfulness in the Christian church also leads to an inevitable disunity. Someday, when Jesus returns, these sins will be eliminated and true unity amongst all God’s people will finally become a universal reality.