What is the whole purpose of God's patience toward lost people?
The whole purpose of God's patience toward lost people is salvation—to bring lost humanity to a knowledge of salvation. The longsuffering of the Lord is salvation, and God is not willing that any should perish but that all should come to a knowledge of salvation.
Les explains that this is the whole purpose of the Book from cover to cover: to bring lost humanity to a knowledge of salvation. He emphasizes Peter is reiterating that the whole scope of God's patience is to bring the lost man to a place of salvation. The text says the longsuffering of our Lord is salvation, and God is not willing that any should perish but that all should come to a knowledge of salvation. Les also notes Peter writes by inspiration of the same Spirit, even referencing beloved brother Paul.