He has Established His Sovereignty over all, men, devil, and death; established it in every realm
This was the testimony being brought home those forty days; His sovereignty in all these things. He was making them know the literalness of His Person in resurrection to be equal to that in the days of His flesh: He was not a spirit, but literally as real in Person as before His death. Pentecost is the climax to the literalness of the resurrection of the Lord Jesus.
Next, the emancipation from the limitation of time and space. Still training an instrument for future usefulness! He is no longer bound by limitations of time. He is seeking to bring home to them that, though not seen, His absence is never to be taken for granted. He is there all the time, and there is no time when He is not with them!
Next, the establishment on the ground of their world commission. Forty days is the period of probation which ends in a demand to assume responsibility. After forty years in the wilderness, the Israelites were commanded to occupy the land.
The Third Part of the Training - the Ten Days after His Ascension
Probation and training issue in the taking up of responsibility. Ten is the number of responsibility. See the boards of the Tabernacle ten cubits high and covered with gold. Here you get the type and teaching concerning man's responsibility towards God; man is only able to meet that responsibility (but he is able to meet it!) on the ground of redemption and in the power of the Divine nature.
Ten days were spent in praying, entering into this thing by prayer, busy with no other burden! And oh! with all the revelation He has given us, should it not bring us to our knees in prayer? Pentecost did not come one day short of full prayer preparation; forty days of revelation and then ten days' prayer; a clear manifestation of Himself and His new thing; and the first carrying of that responsibility in ten days of prayer.
And what was the result of that praying? A thrusting out by and in the power of the mighty Holy Spirit, resurrection revelation, a seeing the fields white unto harvest, and then a giving of themselves unto prayer, until sent forth of the Spirit. Vision, prayer, go; yes, that is the Lord's order.
Here is a threefold preparation, and Pentecost is the climax to that preparation, and the ground upon which the Holy Spirit comes.
"Ye are witnesses of these things ... but tarry Ye ... until ye be clothed with power from on high" (Luke 24:48, 49).
"He was received up, after that He had given commandment through the Holy Spirit unto the apostles whom He had chosen: to whom He also showed Himself alive after His passion by many proofs, appearing unto them by the space of forty days, and speaking the things concerning the kingdom of God: and, being assembled together with them, He charged them not to depart from Jerusalem, but to wait for the promise of the Father, which, said He, ye heard from Me... ye shall be baptized with the Holy Spirit not many days hence" (Acts 1:2-5).
"Then returned they unto Jerusalem ... these all with one accord continued steadfastly in prayer, with the women, and Mary the mother of Jesus, and with His brethren." (Acts 1:12, 14)