Solemnity of Pentecost
And suddenly there came a sound from heaven, as of a mighty wind coming: and it filled the whole house where they were sitting. And there appeared to them parted tongues, as it were of fire: and it sat upon every one of them. And they were all filled with the Holy Ghost. (Acts of the Apostles 2:2-4)
On the fiftieth day, the feast of Pentecost, which was a Sunday that year, the Holy Spirit came down in visible form upon the Apostles confirming them in the truth of the Gospel. Until this time, the Apostles were rather timid and fearful. They were not yet perfect, but were still growing in the faith.
While they themselves certainly believed in the truth of the Resurrection, the Apostles lacked the requisite strength to preach this truth openly and to defend the Gospel against every assault of the world and the devil.
In a word, we may say that the Apostles were not yet confirmed. Through the divine power and the external sign of tongues of fire, our Savior conferred upon the Twelve the reality of the sacrament of Confirmation.