As the Order knows full-well, only the pure of heart can cast a Patronus. When Snape saves Harry from the dementors and reveals his doe Patronus, the Order have reason to trust their spy. Snape knows that he could aid the war effort with his logic and ingenuity, but will the Order trust him enough to accept it? Order of the Phoenix, Snape-centric AU

How many lives could be saved and changed if the Order found reason to trust their spy? When Snape saves Harry from the dementers in the boy's fifth year, he is forced to reveal his Patronus form. And, as the Order well knows, only the pure of heart can cast a Patronus.