Summary:
- He followed all the laws of kosher diet.
- He abstained from anything made from grapes as required for a Nazirite vow.
- He seems to have lived off the fallow land, at least during the times he was baptizing by the Jordan, as Elijah had done for a time, relying on God to provide for him day by day.
- Though this is never specifically mentioned in the gospels, he was a fully active priest, and therefore would have eaten meat and bread made from grain brought in sacrifice while on rotation in the Temple, as required by Torah.
- When not working in the Temple, he abstained from bread. This could be from all bread or only from leavened bread (chametz).
From the gospels and the Torah we learn about several restrictions on the diet of Yochannan (John) the Baptist.
First, as for any Jew, he had to follow the laws of clean and unclean foods. Both locust insects and natural bee honey are considered kosher today in mainstream Judaism. The law permits eating locusts specifically (Leviticus 11:22) but not other "creeping things". Bee honey was ruled kosher by the elders and this conclusion is now Rabbinic tradition.
However, these foods, "locusts and wild honey" that John ate could also be widely misunderstood. These foods, plus his "camel hair" (sackcloth) clothing, are often used to style John the Baptist as a crazy hermit; but I object to this characterization.
There are indications that 'locusts' actually refers to Carob fruit. The word translated "locust" in Latin is locusta, in Greek akrides and in Aramaic qamsa, all literally meaning the locust insect. But this still could just be the nickname for a fruit that does rather resemble a locust - judge for yourself. There is also the question of when live locusts, or carob fruit, would be available. I have no idea.
Regarding honey, even though Rabbinic tradition (codified in the Mishnah, the "oral Torah") holds that bee honey is kosher, the written Torah does not say this explicitly. The elders inferred this through interpretation. Jesus, by contrast, taught that the traditions of the elders are not the inerrant Word of God (Mark 7:1-12) - though in some cases he followed the traditions, such as in the Passover meal. John seems to have believed the same as Jesus did regarding the traditions. However, there are references to bee honey in Tanakh that seem to imply it is not unclean. Perhaps most definitive is Psalm 19:9-10:
The decrees of the LORD are firm, and all of them are righteous. They are more precious than gold, than much pure gold; they are sweeter than honey, than honey from the honeycomb.
It would be strange to liken God's decrees to a substance unclean for food. And how does the psalmist know how honey from the honeycomb tastes, if it is unclean?
I believe that the apostles referred instead to date honey for John's diet. Roman-occupied Judea was famous for its dates in that time and even featured date palms on coins. So dates would have been available. In the Aramaic text of Matthew 3:4 we see ve'devsha d'bara translated as 'and wild honey'. I'm no Aramaic expert, but devsha [DBSA] is spelled almost exactly like modern Hebrew dvash [DBS] which is the word for date honey according to Wikipedia. Looks like a match to me. Also, in the Torah the famous description of the Promised Land, "flowing with milk and honey", referred to date honey not bee honey.
I fancy that "locusts and wild honey" may have been his favorite food - carob chocolate with date honey on top. Sounds yummy! Definitely not the choice of a crazy hermit. Maybe I'll get the chance to try it myself someday! But that's only my own conjecture of course. The point of all this is, whether his favorite food was locust insects covered in bee honey (which I will NOT be trying myself), or carob fruit with date honey, Yochannan ate kosher.
Secondly, Yochannan was a priest. This is known because his father Zakarya (Zechariah) was a priest of the order of Abijah (Luke 1). There are some additional dietary laws specific to the priestly order, mostly concerning the eating of meat and grain brought in sacrifice.
Thirdly, Yochannan lived under a Nazirite vow from birth until at least the day he baptized Yeshua (Jesus) in the Jordan river. This is clearly implied by the instructions given by the Messenger (angel) to his father Zakarya in the Temple, that he should not partake of grapes (Luke 1). So anything containing grapes or any product of grapes was additionally forbidden to him. The messenger actually said, "wine or strong drink", implying other forms of alcohol were also forbidden to him, even though Torah does not explicitly require this for the Nazirite.
But these are only the restrictions in law. It could also be asked, why not eat fish, which are perfectly kosher? Or why not eat the unharvested grains in the farmers' fields, as allowed under the law (and which Jesus and his disciples are known to have done)?
I humbly suggest that these mentions of what he ate should not necessarily be interpreted to mean that Yochannan ate only these foods exclusively. Rather, this description relates what he was known for eating. It became part of his public image, so to speak.
While in service in the Temple, he would have eaten whatever meat and grain was brought in sacrifice, as well as the Temple bread. The law requires that, in fact. And we know he was in active service and on the Temple rotation because he, like Yeshua his cousin, had recently reached the age of thirty, when priests are commanded to enter full duty (Numbers 4:3).
It must be mentioned that multiple resemblances between Yochannan and the prophet Elijah are evident and were called to attention by Yeshua. He wore sackcloth ("camel hair") as Elijah did, symbolizing repentance. He came to the river Jordan in possibly the exact place where Elijah was taken up in a chariot of fire. His diet of "locusts and wild honey" may also have reflected Elijah having relied directly on God to provide his food in the wilderness day to day, living off the land when not serving in the Temple.
Finally, in Luke 7:33 we learn indirectly that Yochannan did not eat bread. The Aramaic text has chama here which resembles the Hebrew for leavened bread (chametz). The same root word is used in the parable of the leaven in Luke 13:21, but not in Luke 22:1 "the feast of unleavened bread". So this seems to have been an abstinence specifically from leavened bread. This abstinence is mentioned by Jesus only in making another point, so he did not clarify whether this was a lifelong abstinence, or seasonal (only while he was baptizing perhaps), or for some other temporary purpose.
