That he might give light to those who were sitting in darkness As to light and darkness, there are similar modes of expression in Isaiah: such as, "The people that walked in darkness have seen a great light: they that dwell in the land of the shadow of death, upon them hath the light shined," (Isaiah 9:1;) and in many other passages. These words show, that out of Christ there is no life-giving light in the world, but every thing is covered by the appalling darkness of death. Thus, in another passage, Isaiah testifies that this privilege belongs peculiarly to the church alone. "Behold, the darkness shall cover the earth, and gross darkness the people: but the Lord shall arise upon thee, and his glory shall be seen upon thee," (Isaiah 60:2.) But how could it be said that the Israelites, on whose hearts the Lord always shone by faith, were sitting in the shadow of death? I reply, the godly, who lived under the law were surrounded on every side by the darkness of death, and beheld at a distance, in the coming of Christ, the light that cheered and preserved them from being overwhelmed by present death. Zacharias may have had in view the wretched condition of his own age. But it is a general truth, that on all the godly, who had ever lived, or who were afterwards to live, there arose in the coming of Christ a light to impart life: for it even diffused life over the dead. To sit is of the same import as to lie: [1] and so Isaiah enjoins the Church, "Arise, for thy light is come," (Isaiah 60:1.) To guide our feet By this expression Zacharias points out, that the highest perfection of all excellence and happiness is to be found in Christ alone. The word Peace might indeed be taken in its literal sense, which would not be unsuitable: for the illumination brought by Christ tends to pacify the minds of men. But as the Hebrew word slvm, peace, denotes every kind of prosperity, Zacharias intended, I doubt not, to represent Christ as the author of perfect blessedness, that we may not seek the smallest portion of happiness elsewhere, but may rest on Christ alone, from a full conviction that in him we are entirely and completely happy. To this purpose are those words of Isaiah, "The sun shall be no more thy light by day, neither for brightness shall the moon give light unto thee: but the Lord shall be unto thee an everlasting light, and thy God thy glory," (Isaiah 60:19.) But if the mere sight of his Son, while still a child, led Zacharias to discourse in so lofty a strain respecting the grace and power of Christ, before he was born, are not they so much the more ungrateful, who, now that Christ has died, and risen, and ascended to heaven, and sat down at his Father's right hand, speak disrespectfully of him and of his power, to which the Holy Spirit bore testimony, while he was still in his mother's womb? We must bear in mind what I have already mentioned, that Zacharias spake not from himself, but that the Spirit of God directed his tongue. And the child grew This is added by Luke for continuing the thread of the history. First, he mentions that John became strong in spirit: which implies that the great and uncommon excellence of the child gave proof that there dwelt in him a Heavenly Spirit. Next, he tells us, that John remained unknown in the deserts till the day of his showing, that is, till the day on which the Lord had pur-posed to bring him into public view. Hence we conclude, that John, though he was fully aware of his calling, made no advances before the appointed time, but awaited the call of God.

Footnotes

1 - "Estre assis emporte autant comme estre couch, ou veautre."-- "To sit is of the same import as to be lying or wallowing."

To give light - See the notes at Matthew 4:16.
To guide our feet - The figure in these verses is taken from travelers, who, being overtaken by night, know not what to do, and who wait patiently for the morning light, that they may know which way to go. So man wandered. So he became benighted. So he sat in the shadow of death. So he knew not which way to go until the Sun of righteousness arose, and then the light shone brightly on his way, and the road was open to the promised land of rest - to heaven.
This song of Zechariah is exceedingly beautiful. It expresses with elegance the great points of the plan of redemption, and the mercy of God in providing that plan. That mercy is "great." It is worthy of praise - of our highest, loftiest songs of thanksgiving; for we were in the shadow of death - sinful, wretched, wandering - and the light arose, the gospel came, and people may rejoice in hope of eternal life.

To give light to them that sit in darkness and [in] the shadow of death, to guide our feet into the (q) way of peace.
(q) Into the way which leads us to true happiness.

To give light to them that sit in darkness,.... God's elect among the Jews, who were not only in a state of unregeneracy, which is a state of darkness, ignorance, and unbelief; but in the darkness of the legal dispensation, and at this time under more than ordinary darkness and ignorance; having lost the knowledge of the righteousness of God, and of the spirituality of his law, the true sense of the Scriptures, and right notions of the Messiah; being led by blind guides, the Scribes and Pharisees,
and, were as it were also,
in the shadow of death; in a state seemingly irrecoverable, when Christ, the great light arose, and shone upon them; and communicated spiritual light, life, and heat unto them; see Isaiah 9:2 compared with Matthew 4:13 though Christ is also a light, to lighten his chosen ones among the Gentiles, Luke 2:32 but the Jews seem chiefly to be intended here:
to guide our feet into the way of peace; which we knew not: not that he came to teach us how to make our peace with God, but to make peace for us, by the blood of his cross; and so by his Spirit and word, lead us into the true way of enjoying spiritual peace here, and eternal peace hereafter.

(Compare Isaiah 9:2; Matthew 4:13-17). "That St. Luke, of all the Evangelists, should have obtained and recorded these inspired utterances of Zacharias and Mary--is in accordance with his character and habits, as indicated in Luke 1:1-4" [WEBSTER and WILKINSON].