12:51 but rather division At face value, this statement seems to contradict the angels’ announcement of peace at Jesus’ birth (see 2:14 and note). However, Jesus is not denying the reality of messianic peace; rather, He is affirming that His death and resurrection will force people to choose sides. Although the way of Jesus certainly leads to peace in God’s kingdom, not everyone decides to follow Him (compare 19:42).
12:51 Part of Jesus’ mission was to create a division within humankind. Other texts of Luke speak of Jesus bringing peace (2:14; 7:50; 8:48; 10:5, 6; Acts 10:36; Eph. 2:13–17). He offers peace to those who respond to Him.
12:51. Jesus continues to amplify His purpose. Ultimately the mission of the Messiah would eventuate in peace. However, the unbelief that characterized the nation would delay that reality.
12:51 “ ‘Do not suppose that I came to grant peace on earth’ ” See the parallel in Matt. 10:34–39. Even the close family relationships in a Jewish home will experience division over Jesus. There is a priority commitment needed to follow Him! Believers form a new family, the family of God!
Luke 12:51. Do you suppose that I came to send peace on the earth? What Christ has now demanded from his disciples any one of them would reckon it an easy matter to give, if the whole world, with one consent, embraced the doctrine of the Gospel. But as a considerable part of the world not only opposes but fights keenly against it, we cannot confess Christ without encountering the resistance and hatred of many. Christ therefore warns his followers to prepare for battle, for they must necessarily fight for the testimony of truth. And here he meets two stumbling-blocks, which otherwise would greatly have distressed weak minds. The prophets everywhere promise that there will be peace and tranquillity under the reign of Christ. What then were his disciples entitled to expect but that, wherever they went, all would instantly be at peace? Now as Christ is called our peace, (Eph. 2:14,) and as the Gospel reconciles us to God, it follows, that he also establishes a brotherly harmony amongst us. The kindling of wars and contentions in the world where the Gospel is preached, does not seem to agree with the predictions of the prophets, and still less with the office of Christ, and with the nature of the Gospel.
But that peace which the prophets describe in lofty terms, is associated with faith, and has no existence but among the sincere worshippers of God, and in the consciences of the godly. To unbelievers it does not come, though it is offered to them; nay, they cannot endure to be reconciled to God: and the consequence is, that the message of peace excites in them a greater tumult than before. As Satan, who holds a kingly power over the reprobate, is furious against the name of Christ, as soon as the doctrine of the Gospel is proclaimed to them, their impiety, which formerly lay asleep, acquires fresh vigour. Thus Christ, who, properly speaking, is the author of peace, becomes the occasion of disturbances in consequence of the wickedness of men.
Let us hence learn how great is the depravity of corrupt nature, which not only soils a gift so inestimable, but changes it into a most destructive evil. Meanwhile, if tumults arise at the commencement of the reign of Christ, let us not be alarmed at it, as if it were strange or unusual: for he compares his Gospel to a sword, and says that it is διαμερισμὸς, separation. Some think that this is intended to describe the punishment which was inflicted on the despisers of the Gospel, by their rising in hostility against each other. But the context shows, that Christ is here exhorting his disciples to perseverance, though a good part of the world should be at variance with them, and though their voice should be like a war-trumpet to call innumerable enemies to arms.
Ver. 51.—Suppose ye that I am come to give peace on earth? I tell you, Nay; but rather division. But the Master quickly leaves himself and his own sad forebodings. He puts by for a season his own holy impatience and continues his warnings. “I have been dwelling on the troublous times quickly coming on. Do not deceive yourselves, my disciples; the great change about to be inaugurated will only be carried out in war and by divisions in the individual house as in the nation. I bring not peace, but a sword, remember.” And then follows a curious picture of a home torn asunder by the conflict of thought which would spring up as the result of the cross and of the preaching of the cross.
12:51 peace on earth … division. The angels spoke of the coming of peace on earth in 2:14 (and cf. 19:38), and Jesus’s disciples have been sent to proclaim peace (10:5–6); he himself wants to bring peace to Jerusalem (19:42). Compare also the summary of Jesus’s mission in Acts 10:36. This exclamation is therefore perhaps not so much a statement of Jesus’s real purpose, but rather a sad recognition that his message of peace will in fact prove to be one of division, as people take opposite sides in relation to him. This too was already predicted at his birth (2:34–35).
 Calvin, J., & Pringle, W. (2010). Commentary on a Harmony of the Evangelists Matthew, Mark, and Luke (Vol. 1, pp. 468–469). Bellingham, WA: Logos Bible Software.