Was Jesus a Christian, or a Jew, or both, or none?

Jesus's self-understanding

Throughout his life on earth 2000 years ago, Jesus assumed dual nature: perfect Jew (100% man) and Son of YHWH (100% God). Except to a few people (like to his parents, Mary and Joseph) His divine nature was revealed only later, during his public ministry.

This is what He did on earth:

1. He came from heaven as a baby (incarnation),
2. grew up normally as a practicing Jew,
3. at about 30 started preaching about himself as God's salvation accompanied by miracles to prove He came from heaven,
4. taught and empowered the 12 apostles and other disciples to start the Christian religion,
5. let himself to be killed as the one-time sacrifice for the whole humanity (part of that salvation),
6. resurrected bodily as the first fruit of the salvation we will one day receive,
7. ascended back to heaven where He came from,
8. sent His spirit at Pentecost to remain with us until today.

Jesus as the center of Christianity

Let's go back to the dual nature, now considered post-ascension, but which is still valid today.

Technically speaking, a Christian is a follower of Christ. "Christ" is a title of Jesus. A follower of Christ means we follow Jesus "as Christ", which includes (among other things) regarding Jesus as God and Lord (King). Common phrasing includes becoming "slave to Christ". Obviously this means Jesus in his divine nature. Otherwise, it doesn't make sense for God to be slave of oneself, or to be one's own admirer. Hence, your confusion.

But when considering Jesus as 100% man (perfect Jew), he can be seen as the role model, as a "success story", as someone we would all like to be, as one who died in perfect obedience to his God, as one who was tempted but didn't fall, as one who wept and felt pain, as one who prayed and worshiped the great God who created the universe, as one who was awarded resurrection of the body (our hope in the life to come). In this sense, he acted like a Christian, since we need to relate to God just like how Jesus did while on earth. Common phrasing includes being "imitator of Jesus", that "Jesus is our brother", etc.

Notice that Christians usually use "Jesus" when considering him as 100% man and "Christ" when considering him as Son of God. Keeping the terminology and Jesus's dual nature in mind helps prevent confusion which I believe triggered your question.

I hope my answer helps!

Neither. Jesus strongly criticized the spiritual ancestors of modern Jews.

So, first, "Christian" as a word refers to the people who are the followers of Christ; it originally derives from a Greek word that meant "little Christs". As a result, I don't think it would be accurate to describe Jesus as a Christian, since he wasn't one of his own followers - he was the leader that his followers were following to begin with.

Secondly, Jesus wasn't a Jew, as we currently use that word. "Jew" refers to a group of people who are spiritually descended from the doctrines of the Pharisees, who Jesus strongly criticized, and ethnically, either descended from those same Pharisees, or from gentile tribes like the Khazars that converted to their religion. We can see evidence of what Jesus thought of them in passages like the following:

I know that you are Abraham’s descendants. Yet you are looking for a way to kill me, because you have no room for my word. I am telling you what I have seen in the Father’s presence, and you are doing what you have heard from your father.”

“Abraham is our father,” they answered.

“If you were Abraham’s children,” said Jesus, “then you would do what Abraham did. As it is, you are looking for a way to kill me, a man who has told you the truth that I heard from God. Abraham did not do such things. You are doing the works of your own father.”

“We are not illegitimate children,” they protested. “The only Father we have is God himself.”

Jesus said to them, “If God were your Father, you would love me, for I have come here from God. I have not come on my own; God sent me. Why is my language not clear to you? Because you are unable to hear what I say. You belong to your father, the devil, and you want to carry out your father’s desires. He was a murderer from the beginning, not holding to the truth, for there is no truth in him. When he lies, he speaks his native language, for he is a liar and the father of lies. Yet because I tell the truth, you do not believe me! Can any of you prove me guilty of sin? If I am telling the truth, why don’t you believe me? Whoever belongs to God hears what God says. The reason you do not hear is that you do not belong to God.”

John 8:37-47

Ethnically, Jesus was a Judean - a citizen of the country of Judea. Religiously, he was a worshiper of God, though he doesn't seem to have been a member of any of the existing major sects such as the Pharisees or the Sadducees (who ran the Temple), though he was associated with the movement started by John the Baptist. Instead, he went on to found his own sect - the early Christians.

For all practical purposes and for any normal usage Jesus was a Jew.

• All his human ancestors were Jewish, making him unquestionably of Jewish ethnicity.
• He was initiated into the Jewish religion through circumcision.
• He was permitted into the synagogue and asked to read (which a non-Jew would not have been allowed to do)
• He observed the Jewish festivals such as Passover
• He is called a Jew (for example John 4:9). The word used here strictly means someone of Jewish ethnicity.
• He used the Jewish scriptures in teaching

Anybody interacting with him would have considered him a Jew.

But isn't he a Christian?

A Christian means a Christ-follower. Jesus is not a Christ-follower, he is the Christ. He cannot be a Christian any more than the Queen of the United Kingdom can be a British Subject. Also the term "Christian" was not used until many years after his death.

You could perhaps attempt to construct an argument saying that he was not a Jew because he taught religious concepts that were not part of the Jewish faith, but that is an argument that is by no means certain to succeed.

Both.

But definition of terms is important for that answer to work.

Jesus was both culturally & religiously Jewish. He was born of a Jewish woman, He taught from Jewish scripture, observed (written) Jewish customs, and taught that He came to fulfil the Law of Moses (meaning He believed the Law was not a mistake but was given by God).

I'll offer a slightly different definition of a Christian, the one given by Peter: someone who believes Jesus is the Christ (the Messiah), the Son of the Living God.

I suggest that using one of the church creeds as the measuring stick of "what is a Christian?" is unhelpful, not least because there were several centuries of Christians before the ecumenical creeds. There were even Christians before there was a written New Testament.

Jesus believed in Himself and His mission and was therefore Christian, even if the term hadn't been coined yet.

The great cultural irony of the Gospel of Matthew is that it was written to people who were simultaneously Jews & Christians. Matthew's basic thesis is that you can be a good Jew and believe in Jesus. In fact he goes further: if you are a good Jew and believe the Tanakh, you should believe in Jesus, because it prophesied of Him.

This was a time when Jewish & Christian were not mutually exclusive terms.