Did Jesus or 'an expert in the law' first share the greatest commandments?

These are two different situations:

(1) Matthew 22:34-40 (+ Mark 12:28-34)

(2) Luke 10:25-37

In the first situation Jesus said the two greatest commandments and the lawyer agreed with Him (Mark 12:32-33):

So the scribe said to Him, «Well said, Teacher. You have spoken the truth, for there is one God, and there is no other but He. And to love Him with all the heart, with all the understanding, with all the soul, and with all the strength, and to love onés neighbor as oneself, is more than all the whole burnt offerings and sacrifices.»

For this Christ praised the lawyer (Mark 12:34):

Now when Jesus saw that he answered wisely, He said to him, «You are not far from the kingdom of God.»

In the second situation Jesus answered the lawyer's question how to inherit eternal life with the question about Law of Moses (Luke 10:26):

What is written in the law? What is your reading of it?

I've asked this question myself!

There are a few opinions that I've heard, the usual answer appears to be that this may have been one of Jesus' common talking points. It would make sense that Jesus was getting involved in the current debates surrounding the Law. This could mean he engaged in several similar conversations.

Some commentators think that there are 2 different accounts in the gospels (Matt & Mark vs Luke). Some think that all 3 accounts represent different events (Matt vs Mark vs Luke).

Here's a link to a useful commentary I read, Meyer's NT Commentary. I think this provides a brief but fair overview of a few opinions!

Hope this helps!

1. These two events are the same because as you read the previous verses, they have the same events like rising from the dead in both accounts (when you follow the verses in previous situations). So, these two are the same situations.

2. The answer of Jesus about the Greatest Commandment to the teacher of the law comes from the Old Testament. This was written by Moses in Deuteronomy 6:5.

The reason why the teacher of the law asked this question because Jesus answered in the previous situation (The Question about Rising from Death), that people will be raised from the dead and be like angels and will not marry. So, the teacher of the law is trying to know HOW to be saved, and so this question about the Greatest Commandment. The teacher of the law is expecting an answer like "offering animal sacrifices" (outward rules) because that is what was written in the Law of Moses in the Old Testament, and so the reason why he answered this in Mark 12:33. And so, Jesus clarified that to "love God" is the greatest commandment in order to be saved (laws in our hearts not by outward rules). (Hebrews 10:16).

I hope it answered your question.

What we today call the "two great commandments" were well-known in Jesus' day

Both commandments had been previously given by Jehovah.

1^st Great Commandment: And thou shalt love the Lord thy God with all thine heart, and with all thy soul, and with all thy might. (KJV Deut. 6:5)

2^nd Great Commandment: Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the Lord. (KJV Lev. 19:18)

So it depends on what you mean by "shared."

• Jehovah was the first to express the ideas.
• Moses was the first to write the ideas down.
• Whomever taught the lawyers of the New Testament the doctrine were before Jesus or the lawyers.

As I led off, the two great commandments were well-known doctrines of the day. They were not introduced first by either Jesus or the lawyers in the New Testament, but were introduced by Moses via Jehovah in the Old Testament and would have been regularly studied in the rabbinical schools. I believe it is reasonable to assume that both the lawyers and Jesus learned the two commandments as children.

… Which is why Jesus' response to the lawyer in Luke 10:26 makes perfect sense.

However, if what you're asking is, "but in that specific situation (or situations), who introduced the idea first, the lawyers or Jesus?" that I can't help you with. I hope I was a spiritual observer, but that veil has been drawn and I doubt there's enough historical evidence to be definitive.