Answer: Once filled with the Holy Spirit, They began to preach the Gospel of Christ to Jews "from every nation under heaven" who were gathered in Jerusalem for the Jewish feast of the Pentecost.
Explanation:
After Christ ascended into Heaven, the disciples returned to the upper room and spent ten days in prayer.
On the tenth day:
"Suddenly there came from the sky a noise like a strong driving wind, and it filled the entire house in which they were. Then there appeared to them tongues as of fire, which parted and came to rest on each one of them. And they were all filled with the Holy Spirit and began to speak in different tongues, as the Spirit enabled them to proclaim".
Filled with the Holy Spirit, they began to preach the Gospel of Christ to Jews "from every nation under heaven" who were gathered in Jerusalem for the Jewish feast of Pentecost.
