There are several synonyms for the phrase "sign/take the pledge". One of the most common is "commit", which implies a promise or dedication to a specific course of action. Another similar phrase is "swear an oath", which implies a solemn promise made in the presence of witnesses or a higher power. "Promise" is another frequently used synonym, indicating a commitment to follow through on a specific action or behavior. Other options include "vow", "affirm", and "declare", all of which convey some degree of solemnity or weightiness to the act of pledging oneself to a particular cause or belief.