Before ascending into heaven, Jesus instructed His disciples to remain in Jerusalem and wait for the promise of the Father - the Holy Spirit. This is recorded in Acts 1:4-5, 8: "Do not leave Jerusalem, but wait for the gift my Father promised... you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth."
This instruction showed that Jesus wanted them to prepare before proceeding on missions. The disciples were not yet equipped to carry out the Great Commission (Matthew 28:19-20) without the empowerment of the Holy Spirit. They needed to wait in Jerusalem until they received spiritual power from on high before they could effectively preach the gospel and establish the church.
The fulfilment came at Pentecost (Acts 2), when the Holy Spirit descended on the disciples, empowering them to speak in other languages and preach boldly. This preparation was essential for the worldwide missionary task ahead.
The instruction was not about replacing Judas (that happened during the waiting period but was not the purpose of staying), avoiding persecution, or restricting the gospel to Jews. In fact, Jesus explicitly commissioned them to be witnesses "to the ends of the earth," including to non-Jews.