A humble interior openness to the Holy Spirit
In the Acts of the Apostles we read how the Holy Spirit appeared to the first group of the one hundred and twenty disciples on the day of Pentecost, under the form of a violent wind which shook the house, and of tongues like fire which came to rest on each of them.
This outpouring of the Spirit signals the visible birth of the Church: it transformed the fearful, trembling apostles, beginning with Peter, into fearless witnesses of Christ, who would henceforward proclaim, fearlessly and cogently, that the crucified Jesus was risen and alive, and demonstrate the truth of their claim by signs and wonders, up to and including martyrdom.
The outpouring of the Spirit, however, is not an event in the past that is over and done with: Pentecost, as a transformative experience brought about by the Holy Spirit, is still going on in the Church.