Amongst all these "descents of the Spirit," which appear to have been by no means infrequent, there was one which left a deep impression on the nascent Church.[4.15] One day when they were assembled together a thunder-storm arose. A violent wind burst the windows open--the sky seemed on fire. Thunder-storms in those countries are accompanied by wonderful illuminations; the atmosphere is furrowed, as it were, on every side with garbes of flame. Whether the electric fluid had penetrated into the very chamber itself, or whether a dazzling flash of lightning had suddenly illuminated all their faces, they were convinced that the Spirit had entered, and that he was poured out upon the head of each one of them under the form of tongues of fire.[4.16] It was a prevalent opinion in the theurgic schools of Syria that the communication of the Spirit was produced by a divine fire, and under the form of a mysterious glimmering.[4.17] It was believed to have been present at the display of all the wonders of Mount Sinai,[4.18] at a manifestation analogous to those of former times. The baptism of the Spirit hence became also a baptism of fire. The baptism of the Spirit and of fire was opposed to and greatly preferred to that of water, the only form with which John had been acquainted.[4.19] The baptism of fire was only produced on rare occasions; only the apostles and the disciples of the first guest-chamber were supposed to have received it. But the idea that the Spirit was poured forth upon them under the form of strokes of flame resembling burning tongues originated a series of singular ideas, which took firm hold of the imaginations of the period.