William Still, in his Notes on Galatians, drew out a most interesting parallel between the coming of the Son into the world and the coming of the Spirit into our heart (Gal. 4:4-6). Just as the Son came as a babe, so the Spirit's first word is "Abba" (Da-da). Still wrote:

“What is the effect of the Spirit of the Son in our hearts? This is most intriguing of all. He comes as Christ came to the earth, a very babe, the merest bundle of life, to cry like an infant; but it is the authentic cry of the child who knows his Parent. The Spirit of the Son in our heart does not cry there for His Father as if He was lost in that poor human heart, but as one whose spirit has been eternally fused  (not confused) with that human spirit, formerly dead in trespasses and sins; so that as the Son cries to His Father in our heart, so man’s spirit, newborn and quickened, cries also to his (new) Father…Now he awakens to the reality, and makes his first cry to the Father. And, of course, the first articulate cry of a child, irrespective of race or nation, is ‘Ab-ba.’”1

  1. Still, William Notes on Galatians (Aberdeem: Didasko Press, reprinted 1972) pp. 56-57