Despite this separation and Snape's loathing of Lily's eventual husband James Potter, who had bullied Snape at Hogwarts, Snape remained in love with Lily. When Snape revealed the prophecy made by Sybill Trelawney (not knowing, at first, that it was referring to Lily and her family) to Voldemort, Voldemort decided to attack the Potters in an attempt to prevent its fulfilment. Though he asked Voldemort to spare Lily, Snape, still fearing for her safety, went to Dumbledore and begged him to protect the Potters. Dumbledore agreed and ensured that they were placed under the Fidelius Charm.
In return, Snape became a double agent for the Order of the Phoenix against Voldemort, using his powers of Occlumency to hide his betrayal from Voldemort. Even with his efforts to protect her, Snape felt responsible for Lily's death when the Fidelius charm was broken. Despite Harry's strong resemblance to James Potter, the fact that he was Lily's son made Snape protect him throughout the series. Snape demanded of Dumbledore, however, that his love for Lily, his reason for switching sides, be kept a secret. Dumbledore agreed and kept the secret throughout the series.