Sign/take the pledge
Definition 1 of 2
Pro Tip 1/3Historical Origins
Originated from pledges to abstain from alcohol, reflecting a serious commitment to a significant change. 
Candidates for the club's membership are often asked to take the pledge to maintain its standards.
Pro Tip 2/3Total Commitment
Implies a total or absolute commitment, usually involving a lifestyle change or moral stance. 
After witnessing the effects of smoking, he decided to take the pledge and quit for good.
Pro Tip 3/3Public Declaration
Often used in contexts where the pledge is made publicly, possibly in a ceremonial way. 
The entire team took the pledge in front of their fans to play fair and with integrity.