Andrew Bonar preaches about the significance of the Day of Pentecost, emphasizing the outpouring of the Holy Spirit as a fulfillment of God's promise after Jesus' glorification. He contrasts the work of the Holy Spirit in the Old Testament with that in the New Testament, highlighting the greater fullness available to believers today. Bonar illustrates the anticipation of the disciples as they prayed in the upper room, leading to the miraculous arrival of the Spirit, which empowered them to boldly proclaim the Gospel. He stresses the importance of persistent prayer for the outpouring of the Spirit, noting that true transformation and revival come through divine intervention rather than human effort. The sermon concludes with a call for believers to seek the Spirit's presence actively, as the early church did, to experience the same powerful results.