Isaiah 12:1

This is the former part of the hymn of praise which is prepared for the use of the church, of the Jewish church when God would work great deliverances for them, and of the Christian church when the kingdom of the Messiah should be set up in the world in despite of the opposition of the powers of darkness: In that day thou shalt say, O Lord! I will praise thee. The scattered church, being united into one body, shall, as one man, with one mind and one mouth, thus praise God, who is one and his name one. In that day, when the Lord shall do these great things for thee, thou shalt say, O Lord! I will praise thee. That is, I. "Thou shalt have cause to say so." The promise is sure, and the blessings contained in it are very rich, and, when they are bestowed, will furnish the church with abundant matter for rejoicing and therefore with abundant matter for thanksgiving. The Old Testament prophecies of gospel times are often expressed by the joy and praise that shall then be excited; for the inestimable benefits we enjoy by Jesus Christ require the most elevated and enlarged thanksgivings. II. "Thou shalt have a heart to say so." All God's other gifts to his people shall be crowned with this. He will give them grace to ascribe all the glory of them to him, and to speak of them upon all occasions with thankfulness to his praise.

— Matthew Henry's Concise Commentary (public domain)