The Christian who experiences the presence of the Holy Spirit within himself, that is to say within his own spirit, experiences the fact of unity of spirit - that unity of person which joins himself with the Divine Person of the Holy Spirit.
... he that is joined unto the Lord is one spirit. [1 Corinthians 6:17 KJV.]
Thus, without (by record) and within (his own self) he is aware of both the fact and the experience of 'drinking in to one Spirit', 1 Corinthians 12:13, in common with all who have such unity and fellowship with the Deity.
This experience does not result in a division of spirit within, for the Holy Spirit dwells within one's own spirit. True, there may be turmoil but the dichotomy is betwixt one's spirit and one's flesh. Paul makes clear in Romans 7 that 'it is no more I that do it' (the sin that is unwilling) but 'sin that dwells within me'. That is to say it is no more my spirit that does it, but sin that is in the flesh.
I say again, the dichotomy is not between two spirits within but is rather between a union of spirit and the flesh.
For sin dwells in the flesh. And the Spirit lusts against the flesh and the flesh lusts against the spirit, Galatians 5:17.
Thus the Christian who experiences such a unity of spirit with God himself, understands that God is one. That is to say God is one Spirit. And understands that when Jesus says 'I and the Father are one' he is saying, in effect, 'I and the Father are one Spirit.'
The verb is plural. But the union is singular. 'They' (plural) are 'one' (singular).
For Jesus also says that 'God is Spirit' or, to more precisely express the Greek, he says 'Spirit, the God' conveying an equivalence, John 4:24, πνευμα ο θεος [TR undisputed].
The reason I say all this is to convey the foundational truth that spirit may dwell in union with spirit. This is a fundamental concept to grasp.
And if spirit dwell in union with spirit, then that spirit is one.
As Jesus says, 'I and the Father are One', John 10:30, εγω και ο πατηρ εν εσμεν [TR undisputed].
So it is not surprising, and it is not difficult to grasp, that Divine Nature is expressed as a singular since the union of Divine Person is so perfect, so intimate, so agreeable, so delightful in amiability, so singular in purpose, so adamant in unanimity - that it is expressed as a singular.
This union could never be expressed under law, or prior to the coming, in flesh, of the Son of God. It is he who declares the Father. Prior to this revelation there were but hints, brief manifestations, allusions, prophecies, but the reality and the fact was not made known until the coming of Jesus Christ and even then, not publicly declared until his own resurrection, Romans 1:4, declared that he is the Son of God and, thus, revealed the Father.
For God was a father to Israel it is true, but that was only as Creator, only as a privilege to a chosen nation upon earth, chosen out of other nations as a means to give testimony to the coming New Testament. It was no more than that.
But the revelation of the Father in the New Testament, upon the resurrection of Jesus Christ, the Son of God, is that of a begetting Spirit and an indwelling Spirit and is the revelation of a union of Spirit, that as far transcends the temporary demonstration of coming reality in Israel as does the heaven transcend the earth.
However, note that when considered as individual persons, the plural is used :
If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him. [John 14:23 KJV],
ελευσομεθα (we will come) and ελευσομεθα (we will make) are both first person plural. But μονην (an abode) is singular. Two persons, one abode, within the one spirit of him who loves the Lord Jesus and keeps his word.
If, now that he is resurrected and ascended, Jesus Christ, the Son of God, is going to come and the Father, also, is going to come and they (plural) are going to make an abode (singular) with me, then God in two persons is going to abide with me, in a single abode, within my own spirit.
Thus is experienced, in Spirit, the fact of Divine Persons dwelling in a perfection of unity, within the born again believer.
The use of the singular is not exclusive, as here is exemplified.