Isaiah 19:18 - Matthew Poole's English Annotations on the Holy Bible

In that day; after that time, as this phrase is used, Isaiah 4:2, Isaiah 18:7, and oft elsewhere. In the times of the gospel, which are oft noted in the prophets by that very expression. Five cities; a considerable number of their chief cities, a certain number being put for an uncertain. Speak the language of Canaan; profess the Jewish religion, agree with them in the same mind; which is fitly signified by speaking the same language, because out of the abundance of the heart the mouth speaketh. Thus the changing and purifying of a people's lips is used to signify the change of their hearts and lives, Zephaniah 3:9; and praising God with one mouth, to note their unity or consent in the faith, Romans 15:6. Swear to the Lord of hosts: it is well observed by some learned interpreters, that he doth not say swear by the Lord, which is the most common phrase, and which, being one eminent part and act of worship, is put for the whole; but swear to the Lord; which phrase is also used 2 Chronicles 15:14 Psalms 132:2 Isaiah 45:23; and it implies the dedication, or oblation, and yielding up of a person or thing to the Lord, by a solemn vow, or covenant, or oath, as appears by the places now quoted. In like manner God is said to swear to a man, Deuteronomy 26:15, and one man to another, Genesis 21:23, when they oblige themselves by oath to do such or such a thing for them. And therefore what is called swearing to God, Isaiah 45:23, is rendered or expounded bowing the knee (which signifies the subjection of a man's self) to God, and confessing to God, Romans 14:11. One; not one of the five, for they are supposed to be saved in the foregoing clause; but one city, or another city, the sixth city. As divers cities shall be converted and saved, so some other cities shall continue in their impenitency, and be destroyed. Others render this clause thus, one of them shall be called, (or, shall be; for to be called is oft put for to be.) The city of the sun; or, as the Grecians call it, Heliopolis; which the Egyptians called On, Genesis 41:45; which was a very eminent city, and a chief seat of idolatry, being a city of priests, as Strabo reports; and therefore its conversion to the faith was more wonderful.