Similar to Jude, by identifying himself as a servant or slave of Jesus Christ, he is affirming the fact that he belongs to Jesus Christ and serves him as his Master and Lord. It could also be a claim to authority and divine commission. In the Bible, the term servant of God
is often used to signify someone that has been set apart specifically for service to God (e.g., 2 Kings 18:12; 2 Kings 21:8; Joshua 24:29; Judges 2:8; Jeremiah 25:4; Amos 3:7; Romans 1:1; Galatians 1:10; Philippians 1:1; James 1:1; 2 Peter 1:1). The apostle Paul talks about Timothy being a servant of the Lord (2 Timothy 2:24) along with Epaphras (Colossians 4:12). It is also used by the apostle John in reference to himself (Revelation 1:1). The fact that Peter also mentions being an apostle underscores his role as one commissioned by God. The authority of Peter thus rests on him being someone whom Christ has sent.1
Additionally, the use of the title apostle
might also serve to mark out the letter as being somewhat official rather than friendly.2
1 Simeon Peter, a servant and apostle of Jesus Christ,To those who have obtained a faith of equal standing with ours by the righteousness of our God and Savior Jesus Christ: