The Christian denomination that popularized the New Year's resolution tradition in the 18th century was the Methodists.
The association comes chiefly from Methodist watchnight services, especially the Covenant Renewal Service linked with John Wesley. These gatherings encouraged worshippers to review their spiritual lives, confess shortcomings, and renew their commitment to Christian discipleship as the new year began. The practice gave New Year observances a structured, devotional form.
The broader idea is much older than Methodism. Ancient Babylonians made promises at Akitu, and Romans connected the new year with Janus, the god of beginnings. Medieval Europeans also renewed vows of ideal behavior. Methodism helped popularize a version centered on personal moral improvement and religious commitment in the English-speaking world.
A common mix-up is treating Methodists as the inventors of every New Year promise. They did not create the underlying custom; rather, their watchnight and covenant-renewal practices helped spread a distinctly Christian form of it. The familiar secular tradition later expanded to goals such as exercising, saving money, or quitting a habit.