That verse comes right after 1 Cor. 6:9-10. So you think Paul is saying that only their new nature is free from fornication, idolatry, adultery, homosexuality, sodomy, theft, coveting, drunkenness, reviling and extortion, but in actual practice they still do these things?
Because you said he was speaking of their new nature, not actual practice. That would kind of hint that they were still doing the things in 1 Cor. 6:9-10, but Paul said that those who do such things will NOT inherit the Kingdom of God.
The Church of Corinth had many problems they were dealing with and because so, they were immature, not growing up in their faith.
Inheritance is not the same as entrance. One can be saved and enter the kingdom of God but yet lose out on inheritance because of selfish living.