John 5:7 King James Version (KJV) of the Gospels state "For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one". My question is does this scripture validate the concept of trinity? How is his scripture interpreted in Islam?
The Islamic position is that additions, omissions and distortions have been done to the previous scriptures by the people, and they are not relied upon for any theological purposes because we can't know what is from Allah and what is human corruption; though works of Tafsir (exegesis) and Tarikh (history) quote them as Israelite traditions where they supplement and don't contradict the Quran and Hadith.
Quran 5:14 And from those who say, "We are Christians" We took their covenant; but they forgot a portion of that of which they were reminded. So We caused among them animosity and hatred until the Day of Resurrection. And Allah is going to inform them about what they used to do.
Quran 2:79 So woe to those who write the "scripture" with their own hands, then say, "This is from Allah," in order to exchange it for a small price. Woe to them for what their hands have written and woe to them for what they earn.
Saheeh Bukhari Ibn `Abbas said, "Why do you ask the people of the scripture about anything while your Book (Qur'an) which has been revealed to Allah's Messenger (ﷺ) is newer and the latest? You read it pure, undistorted and unchanged, and Allah has told you that the people of the scripture (Jews and Christians) changed their scripture and distorted it, and wrote the scripture with their own hands and said, 'It is from Allah,' to sell it for a little gain. Does not the knowledge which has come to you prevent you from asking them about anything? No, by Allah, we have never seen any man from them asking you regarding what has been revealed to you!"
Further, the entirety of the gospels being the Injil is a problematic concept, since the Injil is supposed to be revelation of God to Jesus( 3:48 and 19:30 ), whereas the current gospels are claimed to be human recounts of Jesus's life by the disciples.
Trinity, Jesus's divinity and the concept of begotten son etc. are all explicitly rebuked by the Quran as shirk and kufr.
Quran 5:72 They have certainly disbelieved who say, " Allah is the Messiah, the son of Mary" while the Messiah has said, "O Children of Israel, worship Allah, my Lord and your Lord." Indeed, he who associates others with Allah - Allah has forbidden him Paradise, and his refuge is the Fire. And there are not for the wrongdoers any helpers.
Quran 5:73 They have certainly disbelieved who say, " Allah is the third of three." And there is no god except one God. And if they do not desist from what they are saying, there will surely afflict the disbelievers among them a painful punishment.
Quran 4:171 O People of the Scripture, do not commit excess in your religion or say about Allah except the truth. The Messiah, Jesus, the son of Mary, was but a messenger of Allah and His word which He directed to Mary and a soul [created at a command] from Him. So believe in Allah and His messengers. And do not say, "Three"; desist - it is better for you. Indeed, Allah is but one God. Exalted is He above having a son. To Him belongs whatever is in the heavens and whatever is on the earth. And sufficient is Allah as Disposer of affairs.
Quran 2:116 They say, " Allah has taken a son." Exalted is He! Rather, to Him belongs whatever is in the heavens and the earth. All are devoutly obedient to Him,
Let us also mention that the earlier gospels in the new testament; Mark and Matthew; never mention this trinity concept. Any references as Yeshua or Eesa (pbuh) being son of God are only in a metaphorical sense; ie "Our father in heaven" or "son of man".
As already stated in the other answers, what is written in the new testament are not directly from Yeshua (pbuh), but recollections from his disciples and apostles. While prophets do not err or lie, everyone else is fallible. Don't forget that even Simon Peter, whom Yeshua (pbuh) entrusted to establish the church, lied 3 times in succession that he never knew Yeshua (pbuh). You cannot trust anything divine to come from a mouth that has lied. One lie is enough to invalidate any claims of divine authority.
So how do we sort out what is authentic from the new and old testaments? The last prophet Muhammed (pbuh), was sent as a mercy (rahma) to all (Quran 21:107 ). This includes as a mercy to the Nasara (Christians) and the Bani Israel (Jews).
The Quran is a Furkhan, the criterion between truth and falsehood. The ehlul kitab (people who follow previous scriptures) can find the truth of their scriptures by referring to the Quran. In essence, if something in the previous scriptures agrees with what is in the Quran, it is authentic, otherwise not.
Well you answered you own question as you say correctly this is an addition. However, those are not the words of Jesus in any case. If those were Jesus' words, Muslims wouldn't accept them as accurate and/or would prefer his words in his language -Aramaic which Christians haven't preserved. Even then this "oneness" could be the oneness of purpose as John 17:11;21-23 presents that the disciples are also one with Jesus and God the Father - all divine? No the oneness of purpose https://biblia.com/bible/John17.21-23
This passage is from the First Epistle of John, and contains what is known as the ‘Comma Ioanneum’.
It is shown below in capitals:
‘For there are three that bear record IN HEAVEN, THE FATHER, THE WORD, AND THE HOLY GHOST: AND THESE THREE ARE ONE. AND THERE ARE THREE THAT BEAR WITNESS IN EARTH, the spirit, and the water, and the blood: and these three agree in one.’
Anthony and Richard Hanson write:
‘It (the ‘Comma Ioanneum’) was added by some enterprising person or persons in the ancient Church who felt that the New Testament was sadly deficient in direct witness to the kind of doctrine of the Trinity which he favoured and who determined to remedy that defect . . . It is a waste of time to attempt to read Trinitarian doctrine directly off the pages of the New Testament’. (‘Reasonable Belief: A Survey of the Christian Faith; page 171).
The ‘Comma Ioanneum’ is spurious, and yet for centuries the Church insisted it be included in 1 John 5:7-8; on the grounds that it had become official Church teaching.
In 1927, the Holy Office (Guardian of Catholic orthodoxy; and once named the ‘Supreme Sacred Congregation of the Roman and Universal Inquisition’) declared: ‘After careful examination of the whole circumstances that its genuineness could be denied’ (Ludwig Ott: ‘Fundamentals of Catholic Dogma’, page 56).
This is why my Bible (the Jerusalem Bible - a Catholic version) reads: ‘So there are three witnesses, the Spirit, water and blood; and the three of them coincide.’
I hope this helps.