I'm afraid you can't get from Christ's approval of St. Peter's confession as the foundation of His Church to an assertion that accepting that confession is sufficient as a definition of encorporation into Christ's Church.
Arius accepted that Jesus Christ was the Son of God, but misunderstood His Sonship as the Divine Logos being the first creature. Nestorius accepted His Sonship, but divided the Divine Logos from 'the one from the Virgin'. Eutyches accepted Christ as the Son of God, but denied the reality of His humanity. (I will not continue the catalog of classical christological heresies--you get the idea.)
Brother, David. If it were only his Confession of Faith, why did Jesus tell Simon his name would be changed to Rock the first time He met him (John 1)