Were the Corinthian believers immediately mature in their faith after they were saved?
No, the Corinthian believers were not immediately mature in their faith after salvation. They had been saved out of abject idolatry and paganism, but they were not instantly transformed into spiritual giants. They still had many problems, hang-ups, and failures, as Paul addresses.
Les teaches that the Corinthian assembly had been saved out of abject idolatry, paganism, mythology, the worship of gods and goddesses, and gross immorality. They had no concept of biblical morality. But as these people were saved out of that background, they weren't immediately transformed into spiritual giants. They had a lot of problems, hang-ups, and failures. In spite of that, Paul teaches clearly that if the Lord were to come, it wouldn't only be a few of them taken; they would all go. Les notes there are those who teach an outcalling of only spiritual believers, leaving the carnal behind, but that is not what Paul teaches here.