And it shall be said in that day - By the people of God. This shall be the language of exultation and joy which they shall use.
Lo, this is our God - This is the language of those who now see and hail their Deliverer. It implies that such deliverance, and such mercy could be bestowed only by God, and that the fact that such mercies had been bestowed was proof that he was their God.
We have waited for him - Amidst many trials, persecutions, and calamities, we have looked for the coming of our God to deliver us, and we will rejoice in the salvation that he brings.
This is the Lord - This is Yahweh. It is Yahweh that has brought this deliverance. None but he could do it. The plan of redeeming mercy comes from him, and to him is to be traced all the benefits which it confers on man.