Was Jesus a Muslim ? And should we take the new testament as hadith as it is just a narration of Jesus' life by John , Luke , Mark and others ?
Of course. He was a Muslim, a noble prophet and a messenger of ALLAH(SWT). All prophets were Muslim. How can we know?
Abraham was neither a Jew nor a Christian, but he was one inclining toward truth, a Muslim [submitting to Allah ]. And he was not of the polytheists. [ALI 'IMRAN (FAMILY OF IMRAN) - 67]
Or do you say that Abraham and Ishmael and Isaac and Jacob and the Descendants were Jews or Christians? Say, "Are you more knowing or is Allah ?" And who is more unjust than one who conceals a testimony he has from Allah ? And Allah is not unaware of what you do. [AL-BAQARAH (THE COW) - 140]
Even we think in logical, from Prophet Adam(peace be upon him) to Prophet Muhammad(ﷺ), purpose of all of them is that issue for ALLAH(SWT). And, they talked for mercy of ALLAH(SWT). However, because we can't know whether the words belonged to Prophet Isa or not, we never consider them as hadith or something. Because we believe that all writings before Prophet Muhammad(ﷺ) were changed and distorted. You may think how you can know perish the thought! whether noble Quran is changed or not. I know from the verses in Quran
Indeed, it is We who sent down the Qur'an and indeed, We will be its guardian. [AL-HIJR (THE ROCKY TRACT) - 9]
O you who have believed, obey Allah and obey the Messenger and those in authority among you. And if you disagree over anything, refer it to Allah and the Messenger, if you should believe in Allah and the Last Day. That is the best [way] and best in result. [AN-NISA (THE WOMEN) - 59]
Consequently, we can't know and believe reality of the words of the others in this day and age aside from Prophet Muhammad(ﷺ)