This parable teaches that no one may enter the kingdom of God without an invitation from God. An invitation by grace. It also is a warning to heed the invitation when it is heard. Because the invitation does not last forever. The Banquet is free, the invitation by grace, but acceptance carries with it responsibility.
Discipleship is our responsibility.
Otherwise, when he has laid a foundation, and is not able to finish, all who observe it begin to ridicule him, saying, 'This man began to build and was not able to finish. Or else, while the other is still far away, he sends a delegation and asks terms of peace. So therefore, no one of you can be My disciple who does not give up all his own possessions.