Easter is a very special occasion on the Christian calendar. It is the day on which Jesus resurrected from the dead, the day on which he rose from death to life everlasting. The 49-day period between Easter Sunday and Pentecost Sunday is known as Eastertide. Eastertide is, or should be, the most sacred time on the Christian calendar. It is a time when the apostles were told to wait in Jerusalem for the promised Holy Spirit. They could not do the works of God or fulfill their ministry until they had received the indwelling Holy Spirit. That is a testament to the fact that it is the Holy Spirit who does the work of ministry.
The Holy Spirit cleanses and sanctifies the saints of God, and prepares them to do the work of ministry. Without his sanctifying role, their acts and works of righteousness are as filthy rags (Isaiah 64:6-11). They are corrupted with and by the stain of sin and all manner of unrighteousness. They can never heal, seal, or cause the love of God to flow. These acts are incapable of bearing fruit that will last for all eternity. Nor can they bear fruit that is itself capable of producing more fruit. The land becomes progressively barren if the Holy Spirit is not at the core of everything that is done.
When the Holy Spirit infuses humanity, he revitalizes them, makes them fertile, renders them immune to corruption, and ultimately, renders them eternal. He is the only balm for the plague of sin; he is the only balm for the plague of infertility; and he is the only balm for the plague of eternal death. The apostles knew that they could do nothing without him. They also knew that if they did not obey the Lord's command and wait in Jerusalem, as he instructed them, they may have missed being a witness to the greatest miracle of all time, the eternal becoming united with the mortal.
When God enters humanity, humanity once again can embark on its journey to eternity, guided by the eternal Holy Spirit of God. That is the promise of Eastertide, which we again will witness at the Second Coming of Jesus. He will render the mortal, immortal, and the corruptible, incorruptible. He will call us to a high standard of beingness that we have never before experienced. He will render us perfect, as our heavenly Father is perfect. He will make us gods. That is eternity's promise. That is eternity's commitment, and eternity's call to us. That is eternity.