Concise Bible Dictionary:
To become surety for another is condemned in the Proverbs as being unwise: “he that hateth suretiship is sure.” To be surety for a stranger is totally condemned (Prov. 6:1; Prov. 11:15; Prov. 17:18; Prov. 22:26). Many a Christian has suffered by being surety for a friend. It may be difficult to refuse, but it is unrighteousness unless the one who is surety can bear the loss if it should fall upon him.
The Psalmist asks God to be surety for him for good (Psa. 119:122); and the Lord Jesus is made surety of a better testament, or covenant, than that made with Israel (Heb. 7:22). He is the powerful One who is certain of being able to bring to pass in its due time all that is foretold that He will do in carrying out the purpose of God.