Oops, I misspoke, ealgeone. You said Ac. 2.27-28 was the process to become a believer. No, you (and Peter) gave the process to become a Christian. A person doesn’t have to repent to be a believer. Those who heard the first gospel sermon in Ac. 2 were already believers. Peter had convinced them that they had killed the Christ, and when they asked what they needed to do, Peter told them two things: repent and be baptized for the remission of their sins. They were believers before they repented and were baptized, but they didn’t have the remission of their sins, and they weren’t Christians yet, right?
Thanks again for reading, and considering
No. They were not already believers. There were two groups. The believers, Peter and the ones the Holy Spirit fell upon and the unbelieving Jews who were perplexed at what was happening.