64 Another sense may however be given to the phrase, that just as the Father is seen in the Son, so is the Son in the Spirit. The “worship in the Spirit” suggests the idea of the operation of our intelligence being carried on in the light, as may be learned from the words spoken to the woman of Samaria. Deceived as she was by the customs of her country into the belief that worship was local, our Lord, with the object of giving her better instruction, said that worship ought to be offered “in Spirit and in Truth,” plainly meaning by the Truth, Himself.
As then we speak of the worship offered in the Image of God the Father as worship in the Son, so too do we speak of worship in the Spirit as showing in Himself the Godhead of the Lord. Wherefore even in our worship the Holy Spirit is inseparable from the Father and the Son. If you remain outside the Spirit you will not be able even to worship at all; and on your becoming in Him you will in no way be able to dissever Him from God—any more than you will divorce light from visible objects.
For it is impossible to behold the Image of the invisible God except by the enlightenment of the Spirit, and impracticable for him to fix his gaze on the Image to dissever the light from the Image, because the cause of vision is of necessity seen at the same time as the visible objects. Thus fitly and consistently do we behold the “Brightness of the glory” of God by means of the illumination of the Spirit, and by means of the “Express Image” we are led up to Him of whom He is the Express Image and Seal, graven to the like.
Source: De Spiritu Sancto (New Advent)