The answer
According to Tertullian, being a member of the Church is manifested by good deeds. As for St. Clement of Alexandria, it is the spiritual knowledge or the reasonable perception of the truth, through the unceasing contemplation of God.1160
Fr. Tadros Yacoub Malaty
Notes
1160 Eric G. Jay, The Church: Its changing image through twenty centuries, Volume 1 (London: SPCK, 1977): p. 60.