When was the dwelling of the Holy Ghost in the Church first visibly manifested? The dwelling of the Holy Ghost in the Church was first visibly manifested on Pentecost Sunday, when He came down upon the apostles in the form of tongues of fire.
And when the days of Pentecost were drawing to a close, they were all together in one place. And suddenly there came a sound from heaven, as of a violent wind blowing, and it filled the whole house where they were sitting. And there appeared to them parted tongues as of fire, which settled upon each of them. And they were all filled with the Holy Ghost and began to speak in foreign tongues, even as the Holy Ghost prompted them to speak. (Acts 2:1-4)
Further reading: CCC 767-769