Here is a quick group of verses to read and study.
Matt 16:18, Eph 3:10, Eph 5:23, Col 1:24, 1Tim 3:15 and
All of these verses say something about the Church. Once you’ve read these combine them with the following which pertain to unity:
I therefore, the prisoner in the Lord, beg you to lead a life worthy of the calling to which you have been called, with all humility and gentleness, with patience, bearing with one another in love, making every effort to maintain the unity of the Spirit in the bond of peace. There is one body and one Spirit, just as you were called to the one hope of your calling, one Lord, one faith, one baptism,
Only, live your life in a manner worthy of the gospel of Christ, so that, whether I come and see you or am absent and hear about you, I will know that you are standing firm in one spirit, striving side by side with one mind for the faith of the gospel,
If then there is any encouragement in Christ, any consolation from love, any sharing in the Spirit, any compassion and sympathy, make my joy complete: be of the same mind, having the same love, being in full accord and of one mind.
We have a Church established by Christ and the unity of that Church as described in John 17 is to be such “…that the world may believe that you[the Father] have sent me[Jesus]” This is a visible unity and it is of the utmost importance in converting the world to Christ. The better question to your friend would be, "How could it be any other way?