Thank you for the interesting article. Did Martin Luther believe in eternal security?
You're asking a Presbyterian about Lutheran doctrine.
It's my understanding Luther was not as certain of Eternal Security as the later Reformers, likening Christian assurance to the promise of health by a good physician to his sick patient.
Luther waged a righteous and powerful disengagement from all the corruption and confusion that surrounded him, so he can be forgiven for not getting everything exactly correct. 8~)