James D G Dunn: Jesus said ‘Abba’ to God

It is difficult therefore to escape the conclusion that Jesus said ‘Abba’ to God for precisely the same reason that (most of) his contemporaries refrained from its use in prayer – viz., because it expressed his attitude to God as Father, his experience of God as one of unusual intimacy.

― James D.G. Dunn, Jesus and the Spirit: A Study of the Religious and Charismatic Experience of Jesus and the First Christians as Reflected in the New Testament