To Abraham and to his seed If you read these words in close connection with the close of the former verse, there appears to be an improper change of the case. Instead of to 'Abraam kai to spermati, it ought to have been (pros)ton 'Abraam kai to sperma,, as he spake TO our fathers, TO Abraham and TO his seed [1] But, in my opinion, there is no such close connection. Mary does not merely explain who the Fathers were to whom God spake, but extends the power and result of the promises to all his posterity, provided they are the true seed of Abraham. Hence it follows, that the matter now in hand is, the solemn covenant which had been made, in a peculiar manner, with Abraham and his descendants. For other promises, which had been given to Adam, and Noah, and others, referred indiscriminately to all nations. As many of the children of Abraham, according to the flesh, have been cut off by their unbelief, and have been thrown out as degenerate from the family of Abraham, so we, who were strangers, are admitted to it by faith, and regarded as the true seed of Abraham. Let us therefore hold that, in consequence of God having formerly spoken to the fathers, the grace offered to them belongs equally to their posterity; and also, that the adoption has been extended to all nations, so that those, who were not by nature children of Abraham, may be his spiritual seed

Footnotes

1 - Without attempting to make clear to the English reader the nature of this difficulty, which a Greek scholar will readily enough comprehend, it may suffice to say that the words, as he spake to our fathers, should be read as a parenthesis, and the words now under consideration will then be connected in the following manner: So as to be mindful (or, in remembrance) of his mercy to Abraham, and to his seed, for ever. -- Ed.

As he spake to our fathers - That is, He has dealt mercifully with the children of Israel, according as He promised Abraham, Isaac, and Jacob. The promise particularly here referred to is that respecting the Messiah which was now about to be fulfilled; but there is no doubt that there was also included the promises respecting all the other mercies which had been conferred on the children of Israel. See Genesis 22:17-18.
Forever - These words are to be referred to the preceding verse - "in remembrance of His mercy" forever, "as he spake," etc. They denote that the "mercy of God" manifested to His people should be had in everlasting remembrance.
There is a striking similarity between this song of praise by Mary and that spoken by "Hannah," 1-Samuel 2:2-10. There are few pieces of "poetry" - for this is poetry, and almost the only poetry in the New Testament - more beautiful than this. It is the language of a humble, thankful, pious, female heart praising God:
1. For his mercy to her, Luke 1:46-49;
2. For his mercy to all people - his "general" goodness, Luke 1:50-53; and,
3. His special goodness to his people, Luke 1:54-55.

As he (c) spake to our fathers, to Abraham, and to his seed for ever.
(c) Promised.

As he spake to our fathers,.... To David, of whose family Mary was; and to Jacob, or Israel, of whose stock she was; and to Isaac, in whom the seed was to be, called; and particularly,
to Abraham and to his seed for ever: not his natural, but his spiritual seed; both among Jews and Gentiles, to the end of the world; to these God promised this mercy of a Saviour and Redeemer, and to these he performs it, and will to all generations.

As he spake to our fathers--The sense requires this clause to be read as a parenthesis. (Compare Micah 7:20; Psalm 98:3).
for ever--the perpetuity of Messiah's kingdom, as expressly promised by the angel (Luke 1:33).

To his seed - His spiritual seed: all true believers.