John Calvin Commentary Matthew 19:6

"So that they are no more two, but one flesh. What therefore God hath joined together, let not man put asunder." — Matthew 19:6 (ASV)

What God therefore hath joined. By this sentence, Christ restrains the whim of husbands, so that they may not, by divorcing their wives, break apart the sacred knot. And as He declares that it is not in the power of the husband to dissolve the marriage, He likewise forbids all others to confirm unlawful divorces by their authority. For the magistrate abuses his power when he grants permission to the husband to divorce his wife.

But the purpose Christ had directly in view was that every man should sacredly observe the promise he has given. Furthermore, those who are tempted by immorality or wicked dispositions to divorce should reflect within themselves: “Who are you that allows yourself to burst asunder what God hath joined?”

But this doctrine may be still further extended.

The Papists, by devising for us a church separated from Christ the Head, leave us with an imperfect and mutilated body. In the Holy Supper, Christ joined the bread and the wine, but they have dared to withhold from all the people the use of the cup. To these diabolical corruptions, we may rightly oppose these words: What God hath joined let not man separate.