The topstone or chief cornerstone of a pyramid is itself a perfect pyramid.

The other members of the body, then, must be built up into Christ to conform to that chief cornerstone, which is illustrated by the Great Pyramid in Egypt .-- See Isaiah 19:19.
[318]Again this anointed class, Jesus the head and the church his body, is spoken of as a royal priesthood, a holy nation, a peculiar people.
This was foreshadowed by the nation of Israel, particularly by the priesthood in connection with the house of Israel.

The apostle Peter speaking of this class says: "Ye are a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should show forth the praises of him who hath called you out of darkness into his marvellous light: which in time past were not a people, but are now the people of God: which had not obtained mercy, but now have obtained mercy."-- 1 Peter 2:9,10.
[319]The members of the body of Christ, the church, are also designated as his footstep followers; and as such they are called to suffer with him that they might also reign with him.

"For even hereunto were ye called: because Christ also suffered for us, leaving us an example, that ye should follow his footsteps." (1 Peter 2:21) These are also designated as Christians.

It does not mean that because one is a member of some earthly organization called a church he is a Christian or a member of the true church.