On the Day of Pentecost, Jesus poured out His promised Holy Ghost on all those gathered in the upper room in Jerusalem.

"And when the day of Pentecost was fully come, they were all with one accord in one place. And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance." - Acts 2:1-4 KJV

"And there were dwelling at Jerusalem Jews, devout men, out of every nation under heaven. Now when this was noised abroad, the multitude came together, and were confounded, because that every man heard them speak in his own language. And they were all amazed and marvelled, saying one to another, Behold, are not all these which speak Galilaeans? And how hear we every man in our own tongue, wherein we were born? ... We do hear them speak in our tongues the wonderful works of God." - Acts 2:5-8, 11 KJV

The disciples had just received the promised Holy Ghost. Peter was about to use the keys given to him by Jesus and preach the Gospel of the kingdom. (Acts 1:4-5, Matthew 16:19).

In what language was the Gospel preached on the day of Pentecost?

The Jews that were gathered in Jerusalem that day not only knew their native language from the country they were from, but as Jews, they also knew the Hebrew language. As members of the Roman Empire, they probably knew Greek and Latin as well, as these seem to have been the universal languages at that time.

Peter could have preached to all those gathered in either Greek, Latin or Hebrew, and they all would have understood him. Because all those that were gathered were religious Jews, it is most likely he was preaching in the Hebrew language.

The gospel message on the day of Pentecost was preached by Peter, not by the hundred and twenty speaking in tongues. The Bible clearly records the preaching that day, and it was by Peter. (Acts 2:14-40)

“But Peter, standing up with the eleven, lifted up his voice, and said unto them, Ye men of Judaea, and all ye that dwell at Jerusalem, be this known unto you, and hearken to my words:” – Acts 2:14 KJV

Peter’s conclusion at the end of his Gospel message:

“Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call.” – Acts 2:38-39

 "Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls." - Acts 2:41 KJV