William Porcher Miles was a memebr of teh SC state secession convention. Over that convention flew a red flag with a blue cross of St. George adorned with white stars.
When he became the chairman of the Committee on Flag and Seal for the Provisional Confederate Congress, he was in charge of all submissions for the first flag of the CSA. The vast majority (I have seen all of the submissions) are patterened after the Stars and Srtipes in some way.
The next level of flag submissions had St. George's crosses on them becasue the South was mostly of English descent (not Celtic-Scots as any good geneaologist can tell you as wekll as a number of historians). As these flags got published in the period newspapers (they were BIG news, especially in the two Charleston papers and the Richmond Dispatch), fundamemtalist Christians wrote against it as a mis-use of the corss of their church while the economically and politically powerful Charleston Jews wrote that it was a Christian flag and they would not go for that.
Miles' own design was an attempt to move away from the cross of St. George because of these complaitns and he just tilted the cross into a saltire (which is its proper name in heralddry) and submitted it to his own committee. It had only 7 stars on it and it looked off because of that, so it was shelved. Someone even derisively called it "a pair of blue suspenders," hardly something cheeful if this was some alleged symbol of Celtic-Scots heritage, which it was but not to Miles who was of French Hugenot descent and an Episcopalian too boot (his brother was SC state bishop).
So when the generals were takling of a battle flag for the then CS Army of the Potomac Miles offered his design after shooting down another design given by the commander of the Washignton Artillery Battalion of New Orleans that was sent in by Edward Hancock of that city. Hancocks' design, which rests in Memorial hall in NO today, was a red flag with a blue St. George's cross and the state coat of arms in the upper left corner (for each respective state).
Miles countered that he had been down this road already and cited the religious objections to this flag and that's when he offered his design of the tilted cross/saltire (which is what Miles called it by the way) and, since Missouri had been admitted into the CSA by then and that made for 12 stars, the flag looked a lot better.
And the rest, as they say, is history.
Miles stated that there was no ecclesiastical meaning to his design and that it was a compromise to remove religious objections to the cross of St. George submissions.
Hope this helps.