Alpha (? or a) and omega (O or ?) are the first and last letters of the Greek alphabet; used as a title of Christ in the Book of Revelation (written by John in Greek.).
Revelation 1:8 New International Version (NIV)
8 “I am the Alpha and the Omega,” says the Lord God, “who is, and who was, and who is to come, the Almighty.”
Revelation 21:6 New International Version (NIV)
6 He said to me: “It is done. I am the Alpha and the Omega, the Beginning and the End. To the thirsty I will give water without cost from the spring of the water of life.
Revelation 22:13 New International Version (NIV)
13 I am the Alpha and the Omega, the First and the Last, the Beginning and the End.
The combination of the A & O is the Christian declaration that Jesus is God, having existed before time, and thereafter -eternal. The Almighty!
This symbol has been uniquely incorporated as a variation of the National Association of Christian Ministers logo as a celebration of our faith in Christ alone as our salvation.