Repent and be baptized in the Name of Jesus Christ | Acts 2:38-47: “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. And with many other words did he testify and exhort, saying, Save yourselves from this untoward generation.
Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls. And they continued stedfastly in the apostles’ doctrine and fellowship, and in breaking of bread, and in prayers. And fear came upon every soul: and many wonders and signs were done by the apostles. And all that believed were together, and had all things common; And sold their possessions and goods, and parted them to all men, as every man had need. And they, continuing daily with one accord in the temple, and breaking bread from house to house, did eat their meat with gladness and singleness of heart, Praising God, and having favour with all the people. And the Lord added to the church daily such as should be saved.”
Peter continues to address the apostles and disciples, in which he tells the multitude standing there to repent and be baptized for the remission of sins in the Name of Jesus Christ. Now, Jesus told the apostles right before He ascended (see Acts 1:5) that it would not be water baptism they would receive; however, they would receive a different and new type of baptism, which could be of the Holy Ghost.
Since there were about 3,000 people saved during this encounter, it is highly unlikely (especially in the upper chamber) to have water to do it, and to be able to baptize that many people – all on the day of Pentecost. Therefore, many people were saved and baptized of the Holy Ghost, because they expressed faith in Jesus Christ as Lord and Savior.
Those who were saved continue to learn from the apostles, and fellowshipped with them often. The brethren gathered in each others’ houses and communed, along with several men and brethren bringing friends and family to be saved almost every day.