It has been my understanding that Jesus respnded to teachings within Judaism, so in many cases it pays to know some of the teachings which existed. On this particular subject, I think this might apply.
The Jewish people believed then , as today, that God is One and found it hard to accept Jesus.
   In Rabbinic teaching, it was said ( according to Rashi in commenting on Gen. 49:24 ) that the word "stone or rock"
symbolized the relationship between father & son, which was a sign of permancy. The word for rock is "eben" which is a contraction of Av for father and ben for son.

When Jesus asked in Mat. 16:13,"Who do you say that I am?"
Peter responded, "Thou art the Christ, the Son of the living  God."  Jesus knew that could only be revealed through God and when He said "... upon this "rock" (or "eben" )
I will build my church." I think He might have been referring to the belief in THE FATHER & THE SON.
So I would see it meaning that you must accept Jesus as God's Son to be a part of the church.