John Calvin Commentary Luke 1:36

"And behold, Elisabeth thy kinswoman, she also hath conceived a son in her old age; and this is the sixth month with her that was called barren." — Luke 1:36 (ASV)

And, behold, Elisabeth thy cousin. By an example from her own relatives, the angel encourages Mary's faith to expect a miracle. If neither Elisabeth's barrenness nor her old age could prevent God from making her a mother, there was no better reason for Mary to confine her view within the ordinary limits of nature when she witnessed such a demonstration of divine power in her cousin. He expressly mentions the sixth month, because in the fifth month a woman usually feels the child quicken in the womb, so that the sixth month removes all doubt.

True, Mary should have placed such reliance on God's bare word as to require no support for her faith from any other source; but, to prevent further hesitation, the Lord condescends to strengthen his promise with this new aid. With equal indulgence does he cheer and support us every day; indeed, with greater indulgence, because our faith is weaker. So that we may not doubt his truth, testimonies are brought by him from every direction to confirm it.

A question arises: How could Elisabeth, who was of the daughters of Aaron, (Luke 1:5) and Mary, who was descended from the stock of David, be cousins? This appears to be at variance with the law, which prohibited women from marrying into a different tribe from their own (Numbers 36:6).

With respect to the law, if we look at its purpose, it forbade only those intermarriages that might remove inheritances from tribe to tribe, (Numbers 36:7). No such danger existed if a woman of the tribe of Judah married a priest, to whom an inheritance could not be conveyed. The same argument would hold if a woman of the tribe of Levi married into another tribe.

It is possible that the mother of the holy virgin might be descended from the family of Aaron, and so her daughter might be cousin to Elisabeth.