The first Christian Symbol is the Chi Rho.  It does not belong to any one particular branch of Christianity, but is a universal Christian symbol for all.  The Chi-Rho consists of the intersection of the capital Greek letters Chi (x) and Rho (P), which are the first two letters of the word “Christ” in Greek. 

This Chi-Rho is shown with the letters Alpha (A) and Omega (Ω) the first and last letters of the Greek alphabet. Jesus refers to himself: "I am the Alpha and the Omega, the First and the Last, the Beginning and the End." (Revelation 22:13).