Because of the finished work of Christ

"Up to that time the Spirit had not been given because Jesus had not yet been glorified" John 7:39 (NIV)

John shows in his gospel that Jesus is glorified through his death on the Cross. So when the risen Jesus appeared to his apostles on the first Easter evening, he showed them the marks of his crucifixion, and then breathed on them and said, "Receive the Holy Spirit" – John 20:22 (NIV).

The Spirit is given on the basis of the finished work of Christ.

So Peter tells the Pentecost crowd, "Repent and be baptised, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit" – Acts 2:38 (NIV)

Few have expounded this truth better than Cranmer in the opening words of The Book of Common Prayer’s Eucharistic prayer.
"Almighty God, our heavenly Father, who of thy tender mercy didst give thine only Son Jesus Christ to suffer death upon the Cross for our redemption; who made there (by his one oblation of himself once offered) a full, perfect, and sufficient sacrifice, oblation, and satisfaction for the sins of the whole world …"

We seek the empowering presence and work of the Spirit in our lives and in the lives of other believers on the basis of the complete, sufficient, finished, final, perfect and eternal work of Christ on the Cross.

His death cannot be improved on. His atoning death was perfect at every point. His death both completed the old covenant and initiated the new covenant for the forgiveness of sins.

His death on the cross defeated satan. "He has cancelled the charge of our legal indebtedness that stood against us and condemned us; he has taken it away, nailing it to the cross. And having disarmed the powers and authorities, he made a public spectacle of them triumphing over them by the cross" - Colossians 2:15 (NIV)

"By one sacrifice he has perfected for all time those who are being made holy" – Hebrews 10:14 (NIV)

"I have been crucified with Christ, it is no longer I who live but Christ who lives in me, and the life I now live in the flesh I live believing in the Son of God who loved me and gave himself for me" – Galatians 2:20 (NIV)

"He redeemed us in order that the blessing given to Abraham might come to the Gentiles through Christ Jesus, so that by faith we might receive the promise of the Spirit" - Galatians 3:14 (NIV)

We approach the throne of God on the basis of the completed, perfect work of Christ, his eternal victory. We pray in his name, on the basis of his command, the exhortation of scripture (Joel), his own promise, and on the basis of his completed, finished, perfect and eternal work of atonement.

Probably the clearest articulation of this prayer is by Paul himself …

"I pray that out of his glorious riches he may strengthen you with power through his Spirit in your inner being, so that Christ may dwell in your hearts through faith. And I pray that you … may know the love of Christ" – Ephesians 3:16-19 (NIV).