“Upon this rock I will build my church; and the gates of hell shall not prevail against it.” There need be no mystery or speculation about what rock Jesus meant in this verse, He was simply referring to Isaiah 28:16, “Therefore thus saith the Lord GOD, Behold, I lay in Zion for a foundation a stone, a tried stone, a precious corner stone, a sure foundation: he that believeth shall not make haste.” Jesus is the rock upon which He is building the church.
What is it? The word Jesus used was ekklesia, which means called out. The church is the fellowship of those who have been called out of the world to a new relationship with Jesus and with other followers of Jesus.
Why is it necessary? The values and the goal of those who follow Jesus are different from the values and goals that we formerly had and of those who still do not know Jesus. We need to gather together to help and encourage one another and to be in a place where Jesus can continue to teach us His ways.
“Go ye therefore, and teach (make disciples of ) all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.”
The mission given to the apostles, and to the church, is not simply to make converts, but to make disciples. A disciple is a learner, one who is willing to be taught. We are taught by the Word, by the Holy Spirit and by the church.
Jesus is building the church because we need to be in a place where we can be discipled and disciplined so that we do not lose our way and wind up back where we came from before we met Jesus. That place is where we are all united on the foundation of Jesus Christ and building upon that foundation.