A solemn promise is a commitment made with great seriousness and sincerity. It conveys a sense of steadfastness and loyalty. Some synonyms for the phrase could be a vow, pledge, oath, assurance, covenant, undertaking, commitment, or guaranty. A vow represents a promise made before a higher power, while a pledge is a formal promise made in writing or verbally. An oath is a promise made while swearing on a sacred object or taking a legal obligation. An assurance is a confident declaration of commitment, while a covenant is a solemn agreement between two parties. An undertaking is a commitment made to uphold a responsibility, while a commitment and guaranty both convey a sense of obligation and responsibility.