Numbers 15:15-16 ESV
15 For the assembly, there shall be one statute for you and for the stranger who sojourns with you, a statute forever throughout your generations. You and the sojourner shall be alike before the Lord.
16 One law and one rule shall be for you and for the stranger who sojourns with you.”
We are one because God is one. God lead the Israelites out of Egypt to lead the way for all peoples out bondage of the world. Turning to the Lord makes us alike before the Lord. We are all sojourners in this fallen land.