34 Μὴ νομίσητε ὅτι ἦλθον βαλεῖν εἰρήνην ἐπὶ τὴν γῆν· οὐκ ἦλθον βαλεῖν εἰρήνην ἀλλὰ μάχαιραν. (Matthew 10:34 NA28)
34 Do not think that I came to bring peace on the earth. I have not come to bring peace but a sword. (Matthew 10:34 translated from the NA28 Greek text)
The peace that Christians have because they are in Christ is between them and God and because of that, with each other. However, that does not mean that there will be peace between them and those not in Christ. In fact, these words of our Lord in Matthew 10:34-38 make it clear that those truly in Christ will be so changed by the Gospel, the presence of the Holy Spirit in them, and their new nature that there will be inevitable separation between them and those who are not in Christ. This holds true even within families. This is true because the genuine believer is so changed that no matter what sort of pressure comes to bear on them, they simply cannot be at peace with the ways of the world or the ways of the flesh. Their conscience is bound to Christ and His Word while those not in Christ have consciences bound to the flesh no matter how religious they are.