So the work of redemption was completed on the cross as far as the sacrifice of Christ is concerned.
However, the work of gathering the redeemed goes on, and it continued in that first generation of the church through the apostles, and then through the church as the apostles established the church. This is the history of the good news in the fallen world.
The spel and the establishment of churches passed to Jesus' disciples.  They become the first wave of evangelists and preachers that went out to gather the redeemed and to establish the church.
So this is the history of the church, and this history goes on until the last person in the plan of God is redeemed and the kingdom of God is fulfilled.
A massive undertaking, Jesus commanded his disciples to take the gospel of repentance and salvation to every person on the planet.  That would be beyond our wildest imagination. Just the massive nature of such a task is daunting to even think about, let alone to be assumed, by powerless people ... educationally, academically, religiously.
These are the ones Jesus chose for the job. Disobedient, fickle, yet loyal, disciples.
They were inadequate for the task of the Great Commission. Yet Jesus chose them. He chose them to receive the gift of the Helper, the promise, the Holy Spirit, so that the task of kingdom building through heart transformations of sinners could be accomplished.
Jesus had a 40-day period between His resurrection and His asor them to finish what He started ... to go out and preach the gospel and plant churches in such a way that it extended throughout all of human history and across the globe.
These disciples had to 'wait' until the Father in Heaven sent 'the promise' before they were to go and tell the world the good news. Without this 'Helper' ... their words and actions would be ineffective. The promise was that HIS Spirit would dwell with HIS people ... no longer in a separated holy of holies in the temple ... but HE would dwell within them ... in their hearts ... changing their very souls.