Why wasn't Remus informed that the Secret Keeper had been changed?

Question

Why wasn't Remus informed that the Secret Keeper had been changed? I mean the only ones that knew were James, Lily, Sirius, and Peter in the entire Order. You would think of Remus, as being one of their best friends and a Marauder himself, that they would have informed him. Now I don't know if it was like a full moon or if I read over something in Prisoner of Azkaban, but does anyone have an answer, or at least a theory?

Answer 1

This is discussed by Remus and Sirius when Peter begged them not to kill him in the Shrieking Shack at the end of Prisoner of Azkaban. The reason was that they didn't know who the spy was and Remus was a suspect.

“Remus!” Pettigrew squeaked, turning to Lupin instead, writhing imploringly in front of him. “You don’t believe this ... wouldn’t Sirius have told you they’d changed the plan?”

“Not if he thought I was the spy, Peter,” said Lupin. “I assume that’s why you didn’t tell me, Sirius?” he said casually over Pettigrew’s head.

“Forgive me, Remus,” said Black.

“Not at all, Padfoot, old friend,” said Lupin, who was now rolling up his sleeves. “And will you, in turn, forgive me for believing you were the spy?”

“Of course,” said Black, and the ghost of a grin flitted across his gaunt face. He, too, began rolling up his sleeves. “Shall we kill him together?”

Thus, the reason for not telling Remus was deliberate (they didn't trust him) and not that they simply didn't have time to tell him (as another answer suggests). In fact, we know from Prisoner of Azkaban Chapter 10 that there was at least a full week between the performance of the Fidelius Charm and the deaths of the Potters:

“But James Potter insisted on using Black?”

“He did,” said Fudge heavily. “And then, barely a week after the Fidelius Charm had been performed — ”

“Black betrayed them?” breathed Madam Rosmerta.

“He did indeed.

Answer 2

At the time Sirius was supposed to become secret keeper, he persuaded them at the last minute to change to Peter.

He was pointing at Black, who shook his head slowly; the sunken eyes were suddenly overbright. “Harry . . . I as good as killed them,” he croaked. “I persuaded Lily and James to change to Peter at the last moment, persuaded them to use him as Secret-Keeper instead of me. . . . I’m to blame, I know it. . . . The night they died, I’d arranged to check on Peter, make sure he was still safe, but when I arrived at his hiding place, he’d gone. Yet there was no sign of a struggle. It didn’t feel right. I was scared. I set out for your parents’ house straight away. And when I saw their house, destroyed, and their bodies . . . I realized what Peter must’ve done . . . what I’d done. . . .”

_{Harry Potter and the Prisoner of Azkaban, Chapter 19}

This was done by Sirius in the belief that bluffing Voldemort and his supporters would to come after Sirius was a better option, not knowing that Peter had already sold out to Voldemort.

“Lily and James only made you Secret-Keeper because I suggested it,” Black hissed, so venomously that Pettigrew took a step backward. “I thought it was the perfect plan . . . a bluff. . . . Voldemort would be sure to come after me, would never dream they’d use a weak, talentless thing like you. . . . It must have been the finest moment of your miserable life, telling Voldemort you could hand him the Potters.”

_{Harry Potter and the Prisoner of Azkaban, Chapter 19}

So basically, the only ones that knew were the ones that were there. Peter betrayed the Potters to Voldemort, and made good his escape right after the Potters were killed when Sirius came for him. After that, Sirius was on the run and believed to be responsible, and the only one to gainsay him was apparently dead.

“Just before he transformed,” said Black. “When I cornered him, he yelled for the whole street to hear that I’d betrayed Lily and James. Then, before I could curse him, he blew apart the street with the wand behind his back, killed everyone within twenty feet of himself — and sped down into the sewer with the other rats. . . ."

_{Harry Potter and the Prisoner of Azkaban, Chapter 19}