To give one's word
plight, engage, declare, agree, vow, swear, espouse, consent, asseverate, affirm, profess, undertake, pledge, covenant, contract, bargain, affiance, betroth, assure, guarantee, warrant, give assurance, give warranty, insure, cross one's heart, keep a promise, live up to, plight one's faith, plight one's troth, bind oneself, commit oneself, obligate oneself, make oneself answerable, secure, give security, underwrite, subscribe, lead one to expect, answer for, pledge one's honor, cross one's heart and hope to die*.
Antonyms deceive*, deny, break faith.