“The sacred writing, the divinely inspired scripture clearly and precisely unites to God his Spirit (ie does not make the Holy Spirit inferior to The Father), and openly teaches that in no other way will we participate in God except through the Holy Spirit. And at all events our Lord Jesus Christ said concerning all who are faithful and good, “We will come, I and the Father, and we will abide in him” (Jn 14:23); and “In this we know that he is in us, he gave to us the Spirit” (1 Jn 4:13). For an inhabitation and indwelling of the one who is truly God by nature would greatly enrich someone, but receiving the Spirit that is foreign to him [The Father] would and substantially separated not enrich us, but from Him, in Him, and proper to Him, and bearing the equal lordship to him, and in this way given an equal name and placed at the rank of the Son through natural identity.” St. Cyril of Alexandria, Dialogues On The Trinity 7.64lc-e.
For St. Cyril of Alexandria, our true union with God comes about through the indwelling of the Divine Spirit. This union with God is our salvation (See Here) and thus we celebrate the glorious Pentecost where The Spirit is poured out richly upon us. Through the descent of the Spirit upon us, we have access to The Father and are once more adopted as sons and daughters.