Final answer:
In John Bunyan's 'The Pilgrim's Progress,' the character Temporary backslides before completing his spiritual progress, demonstrating a transient commitment to his journey.
Step-by-step explanation:
The character who backslid before his spiritual progress was completed in John Bunyan's The Pilgrim's Progress is C) Temporary. Temporary is a character who makes some progress on his journey to the Celestial City but later turns back, demonstrating a lack of true commitment to his spiritual journey and the transient nature of his convictions. This contrasts with other characters who, despite obstacles and challenges, persevere towards their spiritual goal.