If Jesus was of the tribe of Judah, with both his parents of the lineage of David, and John the Baptist's parents of Levi's tribe, Luke 1.5, how is it possible that John and Jesus could be related, Luke 1.36?
Because people from different tribes intermarried, a person's tribe was determined by patrilineal descent (their father's father's father etc -- see Numbers 36:7). It is highly probable that more than a millennium and a half after Jacob, all Jews were descendants of multiple sons of Jacob.
For a modern parallel over a slightly shorter timespan, it is believed that Charlemagne is a common ancestor to all people alive today of Western European descent (see here)
The text does not give Mary or Elizabeth's matrilineal line, but the simplest explanation, if the text is reliable, is that Mary was related to Elizabeth on her mother's side.
There are numerous permutations, a few others of which include Mary's paternal grandmother being a descendant of Aaron, Elizabeth's mother being a descendant of Judah, and the list goes on. We don't know for sure, but there's nothing at all improbable about the possibility of John & Jesus being related.
Judah and Aaron were probably both common ancestors to Elizabeth & Mary--it is statistically very improbable that they were not--but in keeping with Jewish custom the text records only their patrilineal genealogy.
The important word in Luke 1:36, usually (correctly) translated "relative" is συγγενής which could be a near or far relative. That is they could have been cousins, second cousins, aunt and niece, etc.
The fact that the tribes intermarried is important here. It means that:
From Jesus earthly father He inherited the right to royal line of David as matt 1 is at pains to document
Jesus Mother was probably via her connection to the priestly family of Zechariah, of the tribe of Levi.
These twin connections are, theologically very significant.
Jesus is King of the Christian Community
The New Testament calls Jesus Christ, Messiah, “the Son of David” as a direct fulfilment of the (ultimately) eternal throne of David which Jesus inherited. Matt 1:1, 20, 9:27, 12:23, 15:22, 20:30, 15, 21:9, 15, Mark 10:35, Luke 1:32, 33, 18:38, 39, John 1:49, Acts 13:32-37, Heb 1:8. See also Rev 11:15, 19:16. Such a Messiah was prophesied long ago: Ex 15:18, Ps 10:16, 61:7, 68:16, 92:8, 93:5, 146:10, Isa 9:7, 47:7, Lam 5:19, Micah 4:7, etc. Compare Isa 55:3 with Acts 13:34.
Note especially, what the angel said to Mary before Jesus’ birth in Luke 1:32, 33 –
He will be great and will be called the Son of the Most High. The Lord God will give Him the throne of His father David, and He will reign over the house of Jacob forever. His kingdom will never end!”
Jesus is now seated on the eternal throne in heaven at the right hand of God, Matt 26:64, Mark 14:62, 16:19, Luke 22:69, Acts 2:33, 7:55-56 (standing), Rom 8:34, Eph 1:20, Col 3:1, Heb 1:3, 8:1, 10:12, 12:2, 1 Peter 3:22. See also Ps 110:1, Matt 22:44, Mark 12:36, Acts 2:34, Heb 1:13, Rev 5.
... but wait there is more ...
Jesus is High Priest of the Christian Community
Jesus is the high priest of the New Covenant Heb 4:14-16, 5:10, 7:23-28, 8:1, 2, 9:1-28, 10:1-18. Thus, Jesus inherits and was the fulfilment of the Levitical Covenant (See Mal 3:1). Jesus did this in several ways as well such as:
- Jesus was the fulfilment of what the sanctuary/temple typified, John 2:19-21, Heb 9:1-28, 10:1-18
- Jesus represented the foundation of the temple as well, 1 Peter 2:4-8 (Compare Isa 28:16, Ps 118:22)
- Jesus’ body was represented by the curtain in the temple, Heb 10:20.
- Jesus was the bread of life, John 6:35, 41, 48 (compare Ex 25:23-30, Lev 24:8).
- Jesus was the light of life, John 8:12, 9:5 (compare the lampstand Ex 25:31-39, Lev 24:3, 4, Isa 53:11, Ps 56:13, etc)
- Jesus provides the water of life, John 4:13, 14 (Compare the laver Ex 30:17-21. See also 1 Cor 6:11)
- Jesus is the promised seed of the woman Gal 3:16 (compare Gen 3:15, and the Abrahamic Covenant)
- Jesus was the Passover Lamb and thus the promised Messiah, John 1:29, 1 Cor 5:7, 1 Peter 1:19 (compare Ex 12:1-14).
- Jesus is the High Priest of the New Covenant in fulfilment of the Levitical covenant, Heb 4:14-16, 5:10, 7:23-28, because He was “pure, blameless, set apart” exactly as the Levites were. See also Heb 9:15, 12:24.
- Jesus provided the blood of the new covenant of which the communion ceremony was to be a memorial, Matt 26:28, Mark 14:24, Luke 22:20, 1 Cor 11:25, Heb 13:20, 1 Peter 1:19 (compare Ex 24:5, 8).
Thus, Mary and Elizabeth were related and thus provided Jesus with the genealogically ideal parents to fulfill His earthly and heavenly mission of being our King and High Priest as Messiah.