'Certain' in the Bible

Now a certain woman, the wife of one of the sons of the prophets, came crying to Elisha and said, Your servant my husband is dead; and to your knowledge he was a worshipper of the Lord; but now, the creditor has come to take my two children as servants in payment of his debt.

Then he went back to the man of God, with all his train, and, taking his place before him, said, Now I am certain that there is no God in all the earth, but only in Israel: now then, take an offering from me.

You may be certain that nothing which the Lord has said about the family of Ahab will be without effect; for the Lord has done what he said by his servant Elijah.