The Lord spoke of His ascent to the Father both in verse 5 of this chapter and in the verse before us, verse 16. In this ascent the Lord indicates that the disciples must place their consolation, for it reveals His unity with God.
Later in the Gospel of John (Jn 20:17, 19-22), we see that the Lord first tells Mary Magdalene that He is ascending to the Father. After this He appears to the disciples and says: "Receive the Holy Spirit."
By this He makes it understood that only when He had co-exalted man with Himself to the throne of God could man on earth receive the gifts of the Holy Spirit in human soul and flesh redeemed and cleansed by the Lord.
Before the mystery of redemption, the disciples were not yet ready to receive the higher meaning of the Lord's words. All they could do at that time was store in their hearts the words of the Lord and God, and await illumination from the Holy Spirit.
This is also indicated in Jn 7:39. For all Christians generally, the Lord's final discourse was revealed, by the command of the Spirit of God, only in the Gospel of John.
As we said in the introduction, this Gospel was the highest completion of the teaching preached in the Church of Christ, giving Christians the fullness of knowledge of the Lord's mysteries.