Charles Ellicott Commentary Deuteronomy 32:4

"The Rock, his work is perfect; For all his ways are justice: A God of faithfulness and without iniquity, Just and right is he." — Deuteronomy 32:4 (ASV)

He is the rock, his work is perfect: for all his ways are judgment: a God of truth and without iniquity, just and right is he. — No such combination of all the words for uprightness, sincerity, equity, and reliability is found elsewhere in all Scripture. This is the character of the Rock. This name of God (Tzur) is one of the characteristics of the song.

The word occurs first in Exodus 17:6, where the Rock in Horeb was struck; and that Rock was Christ. From that time, we find that the very names of the leaders in Israel embody this confession. Elizur, my God is a Rock; Zurishaddai, the Almighty is my Rock; and Pedahzur, redeemed by the Rock (Numbers 1:5–6; Numbers 1:10), are examples. So exclusively is the term in Hebrew (Tzûr or Sêlagh) used in this sense, that no man is ever described by it in the Old Testament.

And the Septuagint, in this song and in many other places, does not translate it at all, but renders it as God (Θεός). In other places the word Petra (never Petros) is employed. This fact convinces me that the Petra of Matthew 16:18 could only have been understood by Jews as denoting Deity; and that it not only referred to Christ, but to Christ as God. No other interpretation will suit the language of Holy Scripture. This fourth verse, like the third, is a stanza of four lines. The first line is answered by the third, and the second line by the fourth.