The Quran

يا أَيُّهَا الَّذينَ آمَنوا كونوا أَنصارَ اللَّهِ كَما قالَ عيسَى ابنُ مَريَمَ لِلحَوارِيّينَ مَن أَنصاري إِلَى اللَّهِ ۖ قالَ الحَوارِيّونَ نَحنُ أَنصارُ اللَّهِ ۖ فَآمَنَت طائِفَةٌ مِن بَني إِسرائيلَ وَكَفَرَت طائِفَةٌ ۖ فَأَيَّدنَا الَّذينَ آمَنوا عَلىٰ عَدُوِّهِم فَأَصبَحوا ظاهِرينَ

Muhammad Asad

O YOU who have attained to faith! Be helpers [in the cause of God - even as Jesus, the son of Mary, said unto the white-garbed ones,¹ "Who will be my helpers in God's cause?" - whereupon the white-garbed [disciples] replied, "We shall be [thy] helpers [in the cause] of God!" And so [it happened that] some of the children of Israel came to believe [in the apostleship of Jesus], whereas others denied the truth.² But [now] We have given strength against their foes unto those who have [truly] attained to faith:³ and they have become the ones that shall prevail.

For this rendering of al-hawariyyun, see surah 3, note 42.
I.e., some of them recognized him as a prophet - and, therefore, as no more than a created, human being - whereas others denied this truth in the course of time by regarding him as "the son of God" - and, therefore, as "God incarnate" - while still others rejected him and his message altogether. The fact that the earliest followers of Jesus regarded him as purely human is evident from the many theological controversies which persisted during the first three or four centuries of the Christian era. Thus, some renowned theologians, like Theodotus of Byzantium, who lived towards the end of the second century, and his followers - among them Paul of Samosata, Bishop of Antioch in the year 260 - maintained that the "sonship of God" mentioned in the then-existing texts of the Gospels was purely symbolic, denoting no more than that Jesus was a human being exalted by God. The originally widespread teachings of Bishop Arius (280-326) centred in the concept of Jesus as a mortal man chosen by God for a specific task, and in the concept of God as absolutely One, unknowable, and separate from every created being; this doctrine, however, was ultimately condemned by the Councils of Nicaea (325) and Constantinople (381), and gradually ceased to have any influence on the Christian masses.
I.e., all who truly believe in Jesus as God's Apostle and thus, as a forerunner of the Last Prophet, Muhammad, whose message confirms and expands the true message of Jesus.