Christianity is not a solo sport. God saves us into a family. Jesus prayed for his followers "that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me." (John 17:21, ESV) Our unity is part of our witness.
Ecclesiastes says that "a threefold cord is not quickly broken" (4:12, ESV). Alone we are vulnerable; together, with God at the centre, we are strong. The first church "devoted themselves to the apostles' teaching and the fellowship, to the breaking of bread and the prayers" (Acts 2:42, ESV). Belonging to a local church — under its care, teaching and sacraments — is God's plan for every believer.
3 CORDS exists to strengthen that, not replace it: to help believers across churches pray together, serve together and reach out together. Different churches, one Lord. "By this all people will know that you are my disciples, if you have love for one another." (John 13:35, ESV)
Go deeper: The Gospel Coalition, Theological Vision for Ministry (Foundation Documents) · Sermons from Tim Keller at Gospel in Life.