The story
From my + brother in Christ. The phrase was in use unironically for softening the delivery of a harsh truth and discouraging the shooting of the messenger before it ever was used sarcastically as an internet meme; it pursued those aims by emphasizing spiritual fraternity between speaker and listener (compare the theme of all due respect and you know I love you, but you need to hear the truth).
Etymology from Wiktionary, CC BY-SA 4.0.