Why do they call Prophet Isa “The Spirit of God”?

Question:
Why do they call Prophet Isa “The Spirit of God”? And what is meant when it is said that Allah blew his spirit into Prophet Adam?

Answer:
Although certain characteristics are shared by different individuals or things and can be found in numerous places, but sometimes for certain reasons, a certain thing or person is identified with that characteristic, despite it not belonging exclusively to it. For example, out of all the things in the universe which all belong to Allah, the Ka’bah has been introduced as the ‘House of Allah’. It is along these lines that although all the prophets are associated with the spirit of Allah, Prophets Adam and Isa, because of their special creation, are known by the title of “Spirit of God”. It is as a result of this that Prophet Isa has come to be known as Ruhullah or the “Spirit of God”, without such title bringing the least flaw in his servitude or bringing him any degree of divinity.

Detailed Answer
There is a verse in the Holy Quran that introduces this prophet of Allah’s as such﴿إِنَّمَا الْمَسيحُ عيسَى ابْنُ مَرْيَمَ رَسُولُ اللهِ وَ کَلِمَتُهُ أَلْقاها إِلى‏ مَرْيَمَ وَ رُوحٌ مِنْه﴾
[1] (Jesus, son of Mary, is only a Messenger of God, His Word, and a spirit from Him whom He conveyed to Mary).
It is also relying on this verse that Ruhullah or the “Spirit of God” has been counted as one of his names in Islamic literature.
On one hand, we must note that the issue of the spirit is one that, according to the explicit assertion of the Quran, we cannot understand entirely and encompass with the limited human intellect we bear [2]. Nevertheless, we see many different verses in the Quran that give numerous meanings for the term “Ruh” or spirit, all of which share a common theme. Here are some examples of the meanings mentioned in the Quran:
1. A proximate angel of Allah’s who delivers Allah’s message to the prophets. [3]
2. A divine book. [4]
3. Aid from the unseen from Allah to His servants. [5]
4. One of the characteristics of Prophet Isa.
Regarding this final meaning, we see that the Quran once names Prophet Isa as a spirit of Allah’s [6], and in another instance, says that he was supported and backed by the Ruh al-Qudus.[7] This difference in wording can be because of the different dimensions of the spirit which isn't very clear to us.
All of what was said might not give rise to any particular question in anyone’s mind; it is certain beliefs of the Christian faith that causes questions to rise regarding this title of Prophet Isa, namely the trinity [8]. The Christians would use these verses to convince the Muslims that even the Quran attributes certain things to Prophet Isa that eventually prove what they believe in, such as him being deity and not the creation of God and rather, the son of God.
It should be known that this matter has existed from a long time ago and our infallible imams have given us appropriate answers to them. Here are two examples:
1. Hamrān asked Imam Sadiq (as) for an explanation on the verse﴿وَ رُوحٌ مِنْه﴾The imam (as) replied: “What is meant by “روح” here is a creation that Allah (swt) has embedded in Isa. [9]
By saying this, the imam implied that just because anyone has the spirit of God in him doesn’t necessarily mean he will also become like his lord.
2.On another occasion, in response to Muhammad bin Muslim in this regard, the sixth imam (as) says: “The same way the lord chose a house from all the houses and named it ‘His’ house, He chose [Ibrahim] from all the prophets and announced him as his ‘friend’ and khalil, and just like other similar cases, He called Isa ‘His spirit’ because he had chosen his spirit from all the other spirits, and this isn't the least in conflict with his createdness; he was just another prophet [like the rest of the prophets] and would carry out His [God’s] will.[10]
Here, the imam (as) is pointing to a very important point, and that is sometimes Allah makes a quality that many share, stand out in a particular individual or thing, but this doesn’t disprove it for the rest. For example:
1. Although this whole universe belongs to Allah [11], He has stressed on a very small area of it (Makkah and the Ka’bah) and used the title ‘My House’ for it [12].
2. Although the Quran unequivocally states that all the prophets and angels are Allah’s chosen ones [13], yet in other places of the same holy book, Allah has touched on specific prophets as his chosen ones.[14]
3. And finally, although divine spirit has been blown into all the prophets, and even believers who aren't prophets, and they are all supported by Allah [15], but this quality has been highlighted in Isa and also before him in Adam [16].
Therefore, we can't see this quality of prophet Isa as something that implies divinity, as if God has incarnated in him.
The first thing Prophet Isa said in the cradle was that he is the servant of the lord [17], and in the end of his mission he says to God that he told his companions to worship the God who is my and your lord [18], “You know whatever is in myself, and I do not know what is in Your Self” [19], once again declaring his servitude to the lord.
This divine messenger was created without a father, the same way Adam was created without a father and mother. Based on this, although the creation of each and every individual is in and of itself a miracle by Allah, but the miracle in the creation of these two prophets stands out more, hence the focus on them by Allah and associating His spirit with them, even though, as was repeatedly mentioned, Allah blows His spirit into whom he desires [20].

Refrence:

[1] Nisa:171.
[2] Isra:85
وَ يَسْئَلُونَکَ عَنِ الرُّوحِ قُلِ الرُّوحُ مِنْ أَمْرِ رَبِّي وَ ما أُوتيتُمْ مِنَ الْعِلْمِ إِلاَّ قَليلا
[3] Nahl:102; Maryam:17; Shu’araa:193, etc.
[4] Shura:52.
[5] Mujadalah:22; Baqarah:87.
[6] Nisa:171
وَ رُوحٌ مِنْه
[7] Baqarah:87 and 253
وَ آتَيْنا عيسَى ابْنَ مَرْيَمَ الْبَيِّناتِ وَ أَيَّدْناهُ بِرُوحِ الْقُدُس
[8] Of course some Christian theologians do not consider themselves tritheists, and through certain expositions, try to prove that they are monotheists. It should be noted that true Christians were indeed worshippers of the one God, as verse 64 of surah Aal Imraan states.
[9] Kuleini, Muhammad bin Yaqub, Kāfī, vol. 1, p. 133, hadith 2, Dar al-Kutub al-Islamiyah, Tehran, 1976.
[10] Ibid, hadith 3.
[11] Aal Imraan:189
وَ لِلَّهِ مُلْکُ السَّماواتِ وَ الْأَرْض
and tens of other similar verses.
[12] Baqarah:125
أَنْ طَهِّرا بَيْتِي
Hajj:26
طَهِّرْ بَيْتِيَ لِلطَّائِفينَ وَ الْقائِمينَ وَ الرُّکَّعِ السُّجُود
[13] Hajj:75
اللهُ يَصْطَفي‏ مِنَ الْمَلائِکَةِ رُسُلاً وَ مِنَ النَّاس
[14] Aal Imraan:33
إِنَّ اللهَ اصْطَفى‏ آدَمَ وَ نُوحاً وَ آلَ إِبْراهيمَ وَ آلَ عِمْرانَ عَلَى الْعالَمين‏
Baqarah:130
...مَنْ يَرْغَبُ عَنْ مِلَّةِ إِبْراهيمَ إِلاَّ مَنْ سَفِهَ نَفْسَهُ وَ لَقَدِ اصْطَفَيْناهُ فِي الدُّنْيا
[15] Mujadalah:22
أُولئِکَ کَتَبَ في‏ قُلُوبِهِمُ الْإيمانَ وَ أَيَّدَهُمْ بِرُوحٍ مِنْهُ
[16] Hijr:29; Saad:72
فَإِذا سَوَّيْتُهُ وَ نَفَخْتُ فيهِ مِنْ رُوحي‏ فَقَعُوا لَهُ ساجِدين
[17] Maryam:30
قالَ إِنِّي عَبْدُ الله
[18] Ma’idah:117
ما قُلْتُ لَهُمْ إِلاَّ ما أَمَرْتَني‏ بِهِ أَنِ اعْبُدُوا اللهَ رَبِّي وَ رَبَّکُم
[19] Ma’idah:116
تَعْلَمُ ما في‏ نَفْسي‏ وَ لا أَعْلَمُ ما في‏ نَفْسِک
[20] Ghaafir:15
يُلْقِي الرُّوحَ مِنْ أَمْرِهِ عَلى‏ مَنْ يَشاءُ مِنْ عِبادِهِ
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