solemn promise

“The funeral of the prince of the Chionites was celebrated according to the rites of the country; and the grief of his aged father was alleviated by the solemn promise of Sapor, that the guilty city of”

The History of the Decline and Fall of the Roman Empire

“A vow is a solemn promise made to God to perform or to abstain from performing a certain thing.”

Smith's Bible Dictionary

“When Ninon learned that her son was plunged into despair and fury on account of her rejection of his love, her heart was torn with sorrow and she regretted that she had not at first told him the secret of his birth, but her solemn promise to de Gersay had stood in her way.”

Ninon de L'Enclos the Celebrated Beauty of the 17th Century

“After some excited discussion they were prevailed upon to acquiesce by the solemn promise of the mandarin to make arrangements with the authorities for their return to their own parts, or failing that to send them back at his own expense; besides, the representation that to turn north again would most likely end in capture by the Japanese vessels, through whose present cruising-ground the gale had luckily blown us, had great weight.”

Under the Dragon Flag

“The young man was not aware of his parentage, de Gersay having extracted a solemn promise from Mademoiselle de l'Enclos that she would never divulge the secret of the youth's birth without his father's express consent, a promise which resulted in the most disastrous consequences.”

Ninon de L'Enclos the Celebrated Beauty of the 17th Century