The Holy Spirit is consubstantial with the Son and the Father, because all Three Persons of the Holy Trinity have a common essence or nature, and a common energy or intention glory. This is why wherever Christ is, there is the Holy Spirit, and wherever the Holy Spirit, there is Christ.
We have already emphasized that the Persons of the Holy Trinity are not separate, nor is their work independent. Saint Maximos the Confessor says that the Holy Spirit is active in all people, but in a different way in each one. It works in all people without exception, since they are created by God, Who is able to hold together, provide for and set in motion the natural seeds. In those peoples who are living by Christ it works to make them sons, since they become sons by the energy of the Holy Spirit. And in the deified, those who have made themselves worthy of citizenship in God and the indwelling of Divine power, it works as creative wisdom. Thus the Holy Spirit is active in all, but differently in each, according to their spiritual condition.
In this light we understand that the Holy Spirit was active in the Old Testament as well, in the Prophets, for it was by the power and energy of the Holy Spirit that the Prophets saw the unincarnate Logos (Word) and prophesied about the Incarnate Logos (Word), Christ. We know very well from Orthodox theology that all of God’s revelations in the Old Testament were revelations of the Second Person of the Holy Trinity, the unincarnate Logos (Word). Since the Logos (Word) is not present without the Holy Spirit, it was the Holy Spirit that manifested the unincarnate Logos (Word) to the Prophets and revealed the future mysteries.
Saint Basil the Great says that the Holy Spirit came to the nous of the Prophets and they prophesied the good things to come. A characteristic example is the case of Saint John the Forerunner, who was filled with the Holy Spirit in his mother’s womb while he was an embryo of six months, and, as Saint Gregory Palamas says, it is by the Holy Spirit that the initiation of the age to come is received in his mother’s womb and makes theology about Christ. And indeed as we saw in the feast of the Meeting, it was by the Holy Spirit that St. Symeon the Righteous recognized Christ.
Therefore the Holy Spirit is active in the Old Testament as well, differently from the way it acts in the New Testament, in the Church. For, as we said before, in the Old Testament it pointed out to the Prophets the transgression of the commandments and revealed the coming of Christ, while in the New Testament it makes men sons of God and members of the Body of Christ and guides them to deification (theosis).
The Incarnation of the Son and Logos (Word) of God and the whole work of the divine Economy is dependent on the Holy Spirit. Saint Basil the Great is very expressive on this point. He says that when Christ comes to dwell in the world, the Holy Spirit precedes Him because it announces His coming and reveals His presence. The Holy Spirit is also inseparable from the presence in the world of the incarnate Christ. The working of the powers and the charismas of the cures is by the energy of the Holy Spirit. The demons come out of men by the Spirit of God. The devil is abolished by the presence of the Holy Spirit. Deliverance from sin is by the grace of the Holy Spirit. The dead are raised through the energy of the Spirit…
The unincarnate Logos (Word), the Son and Logos (Word) of God, said by His Prophet Joel: “I will pour out my Spirit on all people, your sons and daughters will prophesy, your old men will dream dreams” (Joel 2:28). Clearly here he is speaking of the coming of the Holy Spirit and the bestowing of the gift of prophecy which will take place on the day of Pentecost. According to Saint Cyril of Alexandria, the prophecy refers to the events that took place at Pentecost, because at that time the Disciples were speaking prophetically, that is to say, proclaiming the mysteries of Christ which had been foretold by the Prophets. In other words, at that time the Disciples understood by the power of the Holy Spirit that all the prophecies which existed in the Old Testament referred to the Person of Christ. In this way they were perfected in knowledge and revelation.
By the power and energy of the Holy Spirit human nature will return to its previous and ancient condition, because it possesses the prophetic gift of grace which Adam had in Paradise (St. Nicodemos the Hagiorite). Actually, seeing Adam’s life in Paradise before the fall, we discern that he had a pure nous and was prophesying. God formed Eve from Adam’s side while he was sleeping. But when he awoke from sleep and saw Eve, he was enlightened by the Holy Spirit and confessed that she had come from his own body: “This is now bone of my bones and flesh of my flesh” (Gen. 2:23).
This means that all who receive the Holy Spirit and are members of the Church are not only to be restored to the previous state in which Adam was, but will rise still higher because they are united with Christ. A person having the Holy Spirit becomes a prophet, acquires prophetic gifts, as we see in the lives of the Saints, knows the mysteries of Christ, sees and experiences the Kingdom of God. By the power and energy of the Holy Spirit the prophetic gift becomes a natural condition in the person. Noetic prayer is an indication of this gift and of the energy of the Holy Spirit.
(Source: The Twelve Feasts of the Lord by Metropolitan Hierotheos of Nafpaktos).