Some are reborn through water and the spirit (John 3:5); others receive baptism in the Holy Spirit and in fire (Matthew 3:11). I take these four things–water, spirit, fire and Holy Spirit–to mean one and the same Spirit of God. To some the Holy Spirit is water because He cleanses the external stains of their bodies. To others He is simply spirit because He makes them active in the practice of virtue. To others He is fire because He cleanses the interior defilement which lies deep within their souls. To others, according to Daniel, He is Holy Spirit because He bestows on them wisdom and spiritual knowledge (Daniel 1:17 & 5:11-12). For the single identical Spirit takes His different names from the different ways in which He acts on each person.
~St Maximos the Confessor