Can Muslims be considered Muslims and Christians at the same time in the sight of God?

The Catholic Church would not call a Muslim a Christian, for the reasons given in other answers already. But it may be interesting to know that there is a nuance here.

The question was …in the sight of God.

The name “Christian” is a very human thing. We like to give names to everything, just like God asked Adam to do. So we divide groups into smaller groups and use words to identify those groups. So we would not call in Muslim a Christian in the sight of humans.

But what would God see?

The catholic church teaches, as stated in Lumen Gentium, paragraph 16, of the Second Vatican Council, that salvation is very much possible and in accordance with the will of God who wants everybody saved, for anyone who, to say it in short, is not a Christian but by no fault of their own.

So it might be very possible that in the sight of God, the question about being a Christian isn’t as relevant as we make it to be in our world.

Can Muslims be considered Muslims and Christians at the same time?

That depends on what definition of "Christian" is being used.

1. If the definition of "Christian" is "someone who believes that Jesus was a prophet", then it's reasonable to consider trinitarians, non-trinitarians and Muslims to all be Christians
2. If the definition is "someone who believes that Jesus is the Son of God and the Savior of the world", then (as other answers have explained), trinitarians and non-trinitarians are Christians, but Muslims are not Christians
3. If the definition is "someone who receives salvation through Jesus Christ", then who is or isn't considered Christian will vary by sect, but Christians generally don't include Muslims in this category, since most Christians believe that salvation requires faith in Jesus as the Savior of the world, not just as a prophet

Christians usually use something along the lines of definitions 2 or 3; we usually don't use definition 1. So most Christians do not consider Muslims to be Christians (and thus most Christians believe that Muslims are not Christian in God's eyes)

Historically, this was more or less the case. For a long time, Islam was treated by Christians as a heretical splinter sect of Christianity. (This view can be found in Dante's Inferno, for example.)

That understanding has changed over time, largely due to the way Islam goes further than even non-Trinitarian Christianity, rejecting the fundamental premise of the New Testament that Jesus is the Son of God. The Quran says in multiple places that Allah has no son, in contexts making it clear that it is specifically answering and rejecting Christian doctrine.

This, more than anything, is responsible for the view that Muslims are not understood to be Christians. While there are some churches that consider themselves Christian despite doctrinal differences with mainstream Christianity that leads other religions to dispute that claim — my own church among them — Islam does not consider itself Christian to begin with, and actively rejects the doctrine that Jesus is the Son of God. (By contrast, the Jehovah's Witnesses, who also reject the notion that Jesus is God and believe he was simply a great prophet sent by God as the Messiah and exemplar, still claim that he is God's "only-begotten Son", and that his life began in heaven before his mortal ministry.)

As a matter of policy for Christianity.StackExchange specifically, we consider any religion to be Christian that makes that claim about itself. But as Islam actively rejects it and does not claim to be a sect of Christianity, but rather a separate branch of the Abrahamic tradition, we accept that self-identification.

Key to your edited question is your statement that “Muslims are non-Trinitarians, but they believe in Jesus and claim that they follow his teachings”. In your main question you ask “Can Muslims be considered Muslims and Christians at the same time in the sight of God?” The main question will be answered after considering the following points.

The first point that must be addressed is whether the Muslim view of Jesus tallies with the Christian view of Jesus. Here are some fundamental questions that must be answered:

[1] Do Muslims agree with what Jesus said about his pre-mortal existence in heaven?

Through him [the Logos or Word of God] all things were made; without him nothing was made that has been made.... The Word became flesh and made his dwelling among us [Jesus]. We have seen his glory, the glory of the One and Only, who came from the Father, full of grace and truth.” (John 1:3, and verse 14)

For you granted him [your Son] authority over all people that he might give eternal life to all those you have given him. Now this is eternal life: that they may know you, the only true God, and Jesus Christ, whom you have sent. I have brought you glory on earth by completing the work you gave me to do. And now Father, glorify me in your presence with the glory I had with you before the world began. (John 17:2-5)

[2] Do Muslims agree with what the Bible says about the resurrection of Jesus?

Jesus said to her, “I am the resurrection and the life. Whoever believes in me, though he die, yet shall he live, and everyone who lives and believes in me shall never die. Do you believe this?” (John 11:25-26)

And you killed the Author of life, whom God raised from the dead. To this we are witnesses. (Acts 3:15)

Because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. (Romans 10:9)

[3] Do Muslims submit to the risen, resurrected and glorified Jesus Christ?

All authority in heaven and on earth has been given to me. (Matthew28:18)

That at the name of Jesus every knee should bow, in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father. (Philippians 2:10-11)

Those words are quoted from the Old Testament book of Isaiah (chapter 45, verse 43) and were originally applied to Yahweh, the God of Abraham, Isaac, Jacob and Moses. Will any Muslim worship the risen, resurrected Jesus as the Son of God? Or do Muslims deny his deity, and continue to claim he did not die and that he is a follower of Muhammad?

Conclusion: You say Muslims believe in Jesus, follow his teachings but do not believe in the Trinity. A better question would be do Muslims acknowledge his divinity, his pre-existence and his resurrection? The problem is that Islam does not accept who Jesus REALLY is. The Jesus of the Bible is not the same as the Isa of the Qu’ran. The Jesus of the Bible stands out over and above every other qu’arnic prophet, including Muhammad.

It is for the aforementioned reasons that no Muslim can be accepted by God as a Christian. You are either one, or you are the other. Attempts to merge Christianity with Islam are actually an attempt to water down Christianity and compromise Christian beliefs. Such an attempt already exists. https://www.gotquestions.org/Chrislam.html

What are specifically Christian teachings

Most religions agree with Jesus's teachings such as loving God, loving fellow human beings, forgiving enemies, almsgiving for the poor and downtrodden, fighting for justice (especially for victims of injustice), and conducting a good and responsible life. But following these ethical principles is not enough for one to be called a Christian.

Many religions are monotheistic. In addition, Islam and Judaism, being the other 2 Abrahamic religions like Christianity, believe in a God that

• operates in history
• sends His prophets to warn humankind
• gives us ethical commandments to be obeyed
• is almighty and all seeing, thus to be glorified, feared, and respected
• will judge everyone after death with the attendant rewards & punishment

But believing in the One God described above is not enough for one to be called a Christian.

Below are specifically Christian teachings about Jesus that according to all Christian groups, including non-Trinitarians (such as the Church of Jesus Christ of Latter-day Saints, Jehovah's Witnesses, Biblical Unitarians and the Swedenborgian Church) are essential for a Christian but are denied by a Muslim:

1. The New Testament is considered scripture (i.e. inspired by God and is thus considered revelation and authoritative), so what the New Testament says about Jesus take precedence over other sacred texts like the Qur'an.
2. Jesus truly died on the cross and was resurrected by God. It was not a resuscitation (where one will die again) because Jesus was resurrected into a glorified body that will never die again, the "first fruit" of the general resurrection given to all faithful Christians after death (1 Cor 15:20-22). This is an extremely important teaching for Christians because this is the basis of our hope to one day to be resurrected bodily just like Jesus.
3. Jesus is the "Passover Lamb" provided by God to take away the sins of the world, a key aspect of Jesus denied by a Muslim. But in addition to taking away the punishment for sins, Jesus in his human nature voluntarily agrees to BE the lamb because Jesus loves sinners, thus demonstrating the character of a true friend (John 15:13) who is compassionating with us who still suffer from the effect of our own sins although we are no longer condemned (Rom 8:1). That is why having faith in Jesus as our Savior (accepting the gift) and becoming Friend with Jesus (by extending Jesus's friendship to others in love, John 15:12-17) is the essence of being a Christian.
4. Baptism and the Lord's Supper are two essential rituals connected to Jesus, His death, His resurrection, and His sacrifice. But since a Muslim doesn't believe those 3 essential aspects of Jesus (#2 and #3 above), both rituals would be meaningless for a Muslim. There are disagreements of what these 2 rituals signify among Christians, but I think all groups agree that a key prerequisite of the rituals is union with Jesus who give us New Life, Light and spiritual gifts (faith, hope, love, etc.) through the indwelling spirit of Jesus (also called the Holy Spirit) in a believer's soul (John 14:15-17). But this key doctrine of the spirit of Jesus indwelling in the souls of believers is denied by a Muslim.
5. Jesus is the head of the church, and the church is his body that also mediates his presence (God with us, "Immanuel"), continuing his bodily presence spiritually in the bodies of Christians after he ascended to heaven in his glorified body. To a Christian's self understanding, this notion of belonging to the spiritual body of Christ is extremely important, because this is the bodily mechanism of spreading love and other spiritual gifts obtained in point #4 above. In contrast, to a Muslim Jesus is no more than God's messenger ("Rasul Allah"), despite being one of the 5 ul al-'azm (arch prophets) second in rank to Muhammad (pbuh), until Jesus comes again shortly before Judgement Day.

CONCLUSION: Because a Muslim cannot believe and practice the above 5 points, a Muslim cannot be a Christian simultaneously, even though the Muslim respects Jesus highly as an arch prophet, believes many miraculous aspects of Jesus as empowered by God, obeys most of his ethical teachings, and anticipates Jesus's second coming.

Did Jesus say that the New Testament is the final and the last revelation?

The following New Testament verses imply that God has already revealed Himself in the fullest extent possible to a human being by taking on the form of a human life, not just prophecy / spiritual presence as in the Old Testament. By this logic, what more is there to reveal?

1. Some of what Jesus said such as John 14:6:

   "I am the way and the truth and the life. No one comes to the Father except through me."

2. What Jesus said to John in a vision as recorded in Revelation (such as Rev 22:12-13:

   "I am the Alpha and the Omega, the First and the Last, the Beginning and the End"

3. Paul's teaching that Jesus is the "image of the invisible God" (Col 1:15)

4. John's teaching that Jesus is the preexisting God the Word who took on flesh (John 1:1-18)

As for revelation of further teachings, very early in the church history there was a sect called Montanism which was rejected by the majority of local churches (each headed by a bishop) not long after, affirming that there will NOT be new prophecies which supplant or correct the teachings codified in the New Testament.

Of course this doesn't prevent later movements such as one started by the 19th century Joseph Smith who claimed that God gave him further revelation (later recorded in the Book of Mormon) to "supplement" the New Testament. The logic was that the majority Christian leaders corrupted Jesus's teaching (Great Apostasy). This seems to be similar logic of Islam as well.

Which one is true? I think it comes down to which interpretation you trust: will you trust the majority and surviving witness of the Apostolic tradition that closed the NT canon and provided the authoritative interpretation of the Bible by figures such as St. Irenaeus, St. Augustine, and other early church fathers? Or will you trust later and minority interpretation?

At any rate, even the non-Trinitarian movements we had so far hold to the 5 Christian specific teachings I described above, which STILL put them miles apart from Islam's teaching about Jesus.

Being a Christian / Muslim "in the sight of God" vs. Christianity

Responding to your comments (paraphrased below):

1. Muslims believe Jesus was muslim. Islam means submitting to God. Jesus and all prophets submitted to God. Also, Islam has the most similar teachings with Christianity compared with other religions. So Muslims consider "following and believing Jesus" a part of being Muslims. I'm not talking about Muslims to be considered Christians in the sight of an ideology called Christianity. I'm talking about in the sight of God. In God's sight, won't Muslims be considered believers and followers of Jesus in the hereafter?
2. It's people who calls "following and believing Jesus" to be Christianity. It's just a name created by people. But God never named Christianity in the Bible. And Christianity is not the name of a religion recognised by Jesus. Could you show the religion named Christianity in the Bible?

Let's address #2 first. Actually the term Christian is used 3 times in the New Testament: Acts 11:26, Acts 26:28, 1 Peter 4:16. Etymologically, Christian means "follower of Christ", who calls Jesus as "Christ", which in turn means seeing Jesus as the "Messiah" based on certain interpretation of the Old Testament. It is a term for self-identification based on our beliefs of who Jesus is. Similarly, there are other terms that Christians used as self-identification to highlight other aspects of what following Christ means: Disciple (of Jesus), Brother (of Jesus), Saint, Believer (in Jesus), Follower of the Way, Friend (of Jesus), etc. (see Wikipedia).

It is true that God never commanded us to call ourselves "Christian". What's more important "in the sight of God" is that we follow Jesus faithfully according to the teachings of Jesus and his apostles as recorded in the New Testament. Christians can agree with Muslims that God is All-Hearing, All-Seeing (Qur'an 42:11, Psalm 139), and that God looks at the heart, NOT what we call ourselves. According to the New Testament, Jesus himself said that true followers are identified by their obedience ("does the will of [God]", Matt 7:21-23, Luke 6:46, Luke 13:25-27).

But HOW do we follow Jesus rightly, truthfully, and faithfully? What does GOD Himself teach us about Jesus? What is God's TRUE WILL to which we need to submit?

Muslims and Christians agree that revelation is needed to discover the answers since we cannot just look at our own hearts to know the answers like Buddhism. That's why Muslims and Christians:

• have sacred texts (Qur'an and Bible),
• look up to certain human teachers to interpret them (Tafsir and Biblical studies)
• consult additional writings that came down to us through the ages (Hadith and Patristics)
• value human contribution to systematize those 2 sources into various Schools of Islamic Theology and various Christian theologies (theory)
• create systems of ethics and jurisprudence (morality): Halakha for Judaism, Sharia for Islam, Canon law for Catholics, etc.
• develop religious practice over time, inculcate and do it in respective communities: Five Pillars of Islam and Christian spirituality (prayers, going to church, service, meditation, etc.)

As you can see above, both Muslims and Christians have developed different practices based on different theories to direct individual Muslims and Christians to follow Jesus faithfully. The whole complex of theory + morality + practice is properly called a "religion", and the term "Christianity" is simply a name for that practice, the same way the term "Islam" is simply a religion's name as well.

But notice that you said Muslims believe Jesus is a Muslim, which I take to mean that if Jesus were to live longer and had the benefit of reading the Qur'an, he would have testified that Islam IS the true practice ("in the sight of God") for submitting to God and that Christianity is an imperfect religion in need of purification from corruption.

Similarly, Christians believe Jesus is a Christian, meaning that Jesus showed us how to submit to God properly through a life of prayer (Jesus taught the Lord's prayer), conducting life following the teachings in the Sermon on the Mount, loving others like the Good Samaritan, following the teachings that his apostles have taught as recorded in the New Testament, ready to sacrifice / suffer for God like he did, etc. Our Jesus would have said that written revelation has ended with the New Testament and subsequent sacred texts (like the Qur'an) are not binding and need to be subject to the New Testament for proper evaluation.

CONCLUSION: At the end of the day, labels don't matter, but distinct labels (like "Christian" and "Muslim") are STILL needed to truly distinguish spiritual practices (compare and contrast) so that followers of Jesus and worshipers of God are not confused and can choose WHICH spiritual practice is trustworthy (using our reason) so we all (Muslims and Christians) can be approved in the sight of God based NOT on self-identification but on how wholeheartedly we DO the religion we individually have chosen.

Similarly for various Christian groups (both Trinitarian and non-Trinitarian), at Judgment Day labels don't matter in the sight of God. It's more important that a Christian follows Jesus wholeheartedly according to the Christian theology they trust best (whether it is Catholic, Orthodox, Anglican, Lutheran, Reformed, Methodist, Pentecostal, LDS, JW, BU, etc.), although as a Trinitarian I think Trinitarian theology (when understood properly, especially by Catholic Thomism) is a more genuine understanding of the relationship between God and Jesus as portrayed in the Bible, more helpful to appreciate what Jesus did on the cross, and more conducive to a Christian's spiritual growth.