St Cyril of Alexandria, Commentary of the Gospel of John
It can easily be shown from examples both in the Old Testament and the New that the Spirit changes those in whom he comes to dwell; he so transforms them that they begin to live a completely new kind of life. Saul was told by the prophet Samuel: The Spirit of the Lord will take possession of you, and you shall be changed into another man. Saint Paul writes: As we behold the glory of the Lord with unveiled faces, that glory, which comes from the Lord who is the Spirit, transforms us all into his own likeness, from one degree of glory to another.
Does this not show that the Spirit changes those in whom he comes to dwell and alters the whole pattern of their lives? With the Spirit within them it is quite natural for people who had been absorbed by the things of this world to become entirely other-worldly in outlook, and for cowards to become men of great courage. There can be no doubt that this is what happened to the disciples. The strength they received from the Spirit enabled them to hold firmly to the love of Christ, facing the violence of their persecutors unafraid.