What does the phrase You're going to hell in a handbasket mean?

It means to deteriorate rapidly, and originated in the US in the early 20th century. There was an earlier variant of this, "going to hell in a hanging basket" back in the 1890s.

It also means going to hell easily and rapidly.

The New Testament in several passages likens the path to the kingdom of God or Eternal Life as a fight. Paul the main writer of the NT said "I have fought the good fight." Matt. 11.12 states "the kingdom of heaven suffers violence and the violent take in by force. Paul wrote to Timothy to "fight the good fight of faith." Thus there are those who do fight the good fight and make it to heaven while others fight and lose faith in the end ending in hell, while those who succumb to all the lure and evils of the world will go unchained and in a handbasket easily delivered to Satan.