The Vatican II Dogmatic Constitution on the Church (aka Lumen Gentium) §1 ¶1 says that

Christ in the Church [as]…a closely knit union with God and of the unity of the whole human race.

To make concept in that sentence a little clearer, what if we wrote it using a little church math:

The Church = {God + (Unity*Humans)}

God is a constant in this equation. The variable of concern is Unity.

To get the Church available to us, we need to increase the unity among the humans within the Church. With this unity — which does not mean uniformity — we can become the light of unity to the world.

Homework

What are you doing to cause disunity? Stop doing that.

What are you doing to promote unity? Keep doing that.