“Brethren, what shall we do?”
Unless, of course, those around Peter don’t count as “believers”...
Peter was witnessing to these people. It wasn't that these people had believed God and then lost their faith. They were becoming born again.
1Jn 2:19 They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us.