What was the "Promise of the Father" that Jesus spoke of in Luke 24:49?

The book of Luke 24:49 takes place in Emmaus, a city that is approximately 60 stadia northwest of Jerusalem. Jesus had already risen from the dead and had given the disciples comfort and instructions concerning his impending departure and the work that they must do as his Apostles. The “Promise of the Father” that Jesus instructed the Apostles to remain in Jerusalem and wait for, is none other than the Holy Spirit, the third member of the “Holy Trinity”.

In the book of Acts chapter 2, we see the Promise of the Father fulfilled. As the Apostles were praying in the upper-room of a residence that the Apostles lived in, the Holy Ghost, in what is one of the more memorable miracles of the Holy Bible, descended from Heaven in the form of “cloven tongues like as of fire,” and rested on the heads of the Apostles (Acts 2:3). The Apostles then began speaking in “tongues” (languages) that they presumably never studied. By the power of this Holy Ghost, the Apostles spoke languages ranging from Grecian to Persian, as well as languages from every territory in between. It is said that the Apostles were saying in foreign tongues, everything that the Holy Spirit told them to say, (Acts 2:4). The Holy Spirit was telling them to preach in foreign tongues of “the wonderful works of God” (Acts 2:11). A small crowd of men from different nationalities soon gathered to where the Apostles were, and were amazed at their fluency in every man’s language. They even at one point said, “are not all these which speak Galilæans?” (Acts 2:7).

Overall, this story is indicative of what the ministry of the Apostles was all about, and that is preaching the Gospel of Jesus Christ to all nations, and kindreds, and people, and tongues (Matthew 28:19, Acts 2:17, Galatians 3:28, & Revelation 7:9).