For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father. – Romans 8:15

There are seasons when doubts and fears abound, and so suffocate us with their fumes that we cannot even raise a cry, and then the indwelling Spirit represents us, and speaks for us, and makes intercession for us, crying in our name, and making intercession for us according to the will of God. Thus does the cry “Abba, Father” rise 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 dwelleth in me.” On the other hand, at times our soul gives such a sweet assent to the Spirit’s cry that it becometh ours also, but then we more than ever own the work of the Spirit, and still ascribe to Him the blessed cry, “Abba, Father.”

God hath sent the Spirit of His Son into our hearts, and that Spirit cries in us exactly according to the cry of the Son. If you turn to the gospel of Mark, at the fourteenth chapter, thirty-sixth verse, you will find there what you will not discover in any other evangelist (for Mark is always the man for the striking points, and the memorable words), he records that our Lord prayed in the garden, “Abba, Father, all things are possible unto Thee; take away this cup from Me: nevertheless not what I will, but what Thou wilt.” So that this cry in us copies the cry of our Lord to the letter-“Abba, Father.” “Abba” is not a word, somehow, but a babe’s lisping. Oh, how near we are to God when we can use such a speech! How dear He is to us and dear we are to Him when we may thus address Him, saying, like the great Son Himself, “Abba, Father.” ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/1435.cfm