St Paul tells us, in his First Letter to the Corinthians,
I want you to understand that on the one hand no one can be speaking under the influence of the Holy Spirit and say 'Curse Jesus', and on the other hand, no one can say, 'Jesus is Lord' unless he is under the influence of the Holy Spirit.
So the work of the Spirit is to create faith in Jesus. Intellectually, St Thomas was unable to accept that Jesus had risen from the dead. He could neither receive nor comprehend this truth. But the Spirit -- breaking down the barriers of prejudice, scepticism and unbelief in Thomas's mind -- gave him the spiritual understanding to truly see the Lord, and to profess his faith in Jesus as Lord and God.
The Greek word used by St John for 'to see' denotes a sense of spiritual understanding beyond intellectual understanding.
To create faith, the Spirit works in a mind that is searching for truth, and is willing to accept it and submit to it. But the Spirit also uses the witness of the believing community to create faith. The faith of the disciples had a powerful influence on Thomas, inclining him to believe. His coming to faith, therefore, was not just a singular experience but something that took place within the new-born Church.
The Spirit, working through the disciples' witness to the resurrection, and through Thomas's desire for truth, created a living faith in that once-doubting Apostle.
The same principle can be seen in the description of the growth of the Church in today's first reading. As people saw the faith of the believing community and the radical changes that this faith brought about in them, the regarded them with goodwill. They were inclined to believe their message.
The Spirit led many of them into living faith. When a body of believers truly believes, others also will be brought to faith through their witness.
In confessing Jesus as 'My Lord and my God', Thomas made the most profound declaration about the person of Christ -- that he is truly God. He accepted Jesus as Lord and submitted his life to Christ's lordship. As we celebrate the resurrection of Jesus we can use Thomas's words to declare our faith and submit our lives to him more deeply.
In addition we can ask the Holy Spirit to deepen our spiritual understanding as we open our minds more completely to the witness of scripture and the Christian community. We can witness confidently to Christ's resurrection, knowing that the Spirit will use our words and witness to bring others to faith, just as Thomas was brought to faith.