The answer is that "Muslim" has both a general meaning and a later community meaning. Jesus can be called a Muslim in the Quranic sense - one who submits to the one God - without claiming he followed the legal code revealed later to Prophet Muhammad (PBUH).

The objection at its strongest

The objection is fair enough to deserve a serious answer. If "Muslim" means a follower of Prophet Muhammad (PBUH), then calling Jesus a Muslim sounds anachronistic. And if "Muslim" merely means "someone who submits to God," the word may seem too broad to prove anything specifically Islamic.

The sharper form is about Jesus' prayer, "not my will but yours," addressed in the Gospel to "the Father." Since Islam rejects calling Allah a father in any literal or divine-sonship sense, the objector says Muslims face a dilemma: either Jesus used blasphemous language, which a prophet cannot do, or Christian sonship language is true and Jesus was not a Muslim.

What the Quran means by "Muslim"

In Arabic, "muslim" means "one who submits." The Quran uses it for believers before Prophet Muhammad (PBUH), not as a historical claim that they prayed the later five daily prayers or followed the later legal details, but that they surrendered to Allah's oneness and obeyed the messenger sent to them.

Abraham was neither a Jew nor a Christian, but he was upright, a Muslim, and he was not of the polytheists.

Quran 3:67

And when I inspired the disciples: "Believe in Me and in My messenger," they said: "We believe, so bear witness that we are Muslims."

Quran 5:111

This is not a trick definition. It is like saying earlier prophets were "monotheists" even if the English word came later. The substance came first; the label can be applied later. At the same time, after the coming of Prophet Muhammad (PBUH), being Muslim includes accepting his prophethood, because he is the messenger now sent to mankind.

The prophets are paternal brothers; their mothers are different, but their religion is one.

Sahih al-Bukhari and Sahih Muslim

Jesus' submission is exactly the point

The prayer "not my will but yours," if accepted as preserving something Jesus said, does not prove divinity. It sounds like submission: a servant yielding to his Lord. That is precisely the Quranic portrait of Jesus.

Indeed, Allah is my Lord and your Lord, so worship Him. This is a straight path.

Quran 3:51

The issue is the word "Father." Islam rejects that Allah is Father in the sense of begetting, divine paternity, shared essence, or literal sonship. That is shirk because it gives Allah a child or partner in divinity.

Say: He is Allah, One, Allah, the Eternal Refuge. He neither begets nor is born, and there is none comparable to Him.

Quran 112:1-4

But the mere occurrence of father-language in a later Gospel text does not force Muslims to say Jesus committed shirk. Muslims do not treat the New Testament wording as a verbatim transcript of Jesus' revealed teaching. And even within Biblical usage, "father" can function as a metaphor for care, authority, and covenantal nearness, not necessarily literal divine sonship. Islam would not endorse addressing Allah this way in our worship, because Allah has taught us His names and how to call upon Him. But a metaphor in an earlier religious idiom, if not meant literally, is not the same as worshiping another god.

What this does and does not prove

Calling Jesus a Muslim does not prove he followed the Sharia of Prophet Muhammad (PBUH), nor does it erase historical differences between prophetic communities. It means he worshiped the one God, submitted to Him, and called others to that worship.

Nor does the Gospel phrase "Father" by itself prove the Christian doctrine of divine sonship. The real disagreement is over whether Jesus taught pure servanthood to Allah or ontological sonship. The Quran answers that directly: Jesus was a noble messenger, not God, and not the son of God. That is why Muslims honor him as a prophet who submitted perfectly to his Lord.