There are seasons when doubts and fears abound, and so suffocate us with their fumes that we cannot even raise a cry. Then the indwelling Spirit represents us, speaks for us, and makes intercession for us, crying in our name, and making intercession for us according to the will of God. The cry "Abba, Father" rises up in our hearts even when we feel as if we could not pray and dare not think ourselves children. Then we may each say, "I live, yet not I, but the Spirit that dwells within me."
On the other hand, at times our soul gives such a sweet assent to the Spirit’s cry that it becomes ours also. Then, at that time, we more than ever own the work of the Spirit, and still call out to him the blessed cry, "Abba, Father."
Notice a very sweet fact about this cry: it is literally the cry of the Son. God has sent the Spirit of his Son into our hearts, and that Spirit cries in us exactly according to the cry of the Son.
Source: Allan Kircher