St Basil the Great
[T]he Spirit... gives us freedom to call God our Father. 'For God hath sent forth the Spirit of His Son into' our 'hearts crying Abba, Father,' that the voice of the Spirit may become the very voice of them that have received him."
- St Basil the Great, On the Holy Spirit, 57
Today, 2 January, is the feast of two great Doctors of the Church, St Basil and St Gregory Nazianzen, who were among the trio known as the Cappadocian Fathers. Both were born in 330, and were friends. Both did much to develop our theology of the Holy Trinity. In particular, St Basil wrote a great treatise 'On the Holy Spirit', using Scripture and Tradition to show the divinity of the Holy Spirit.
Hence the Spirit is shown in the form of a dove hovering beside the saint. This fresco of St Basil is on the ceiling of Sta Maria sopra Minerva, the Domincan church in the centre of Rome.