A.T. Robertson Commentary 1 Peter 5:1

"The elders among you I exhort, who am a fellow-elder, and a witness of the sufferings of Christ, who am also a partaker of the glory that shall be revealed:" — 1 Peter 5:1 (ASV)

Who am a fellow-elder (ο συνπρεσβυτερος). Earliest use of this compound in an inscription of B.C. 120 for fellow-elders (alderman) in a town, here only in N.T., in eccles. writers. For the word πρεσβυτερος in the technical sense of officers in a Christian church (like elder in the local synagogues of the Jews) see Ac 11:30; 20:17. It is noteworthy that here Peter the Apostle (1:1) calls himself an elder along with (συν) the other "elders."

A witness (μαρτυς). This is what Jesus had said they must be (Acts 1:8) and what Peter claimed to be (Acts 3:15; Acts 10:39). So Paul was to be a μαρτυς (Acts 22:15).

Who am also a partaker (ο κα κοινωνος). "The partner also," "the partaker also." See Lu 5:10; 2 Corinthians 1:7; 2 Peter 1:4. See same idea in Ro 8:17. In Ga 3:23; Romans 8:18 we have almost this about the glory about to be revealed to us where μελλω as here is used with the infinitive.