Because you are God’s children, God has sent the Spirit of his Son into us to call out, “Abba! Father!” (Galatians 4:6)
Paul could have said: “God has sent the Spirit of his son into us to pray: ‘Abba, Father.'” But he purposely says “call out” to indicate the anguish of the Christian who is still weak and needs to grow in the faith. In Romans 8:26, he describes this calling out as “groans that cannot be expressed in words.”
But in the middle of trials and conflicts, it’s difficult to call out to God, and it takes a lot of effort to cling to God’s Word. At those times, we cannot perceive Christ. We do not see him. Our heart doesn’t feel his presence and his help during the attack. Christ appears to be angry with us and to have left us. Then during the attack, we feel the power of sin, the weakness of our bodies, and our doubt. We experience the flaming arrows of the devil (Ephesians 6:16) and the terrors of death. We feel the wrath and judgment of God. All this raises very powerful and horrible shouts against us so that there does not appear to be anything left but despair and eternal death. However, in the middle of these terrors of the law, the thundering of sin, the shaking of death, and the roar of the devil, the Holy Spirit in our hearts begins to call out, “Abba! Father!” And his cry is much stronger and drowns out the powerful and horrible shouts of the law, sin, death, and the devil. It penetrates through the clouds and heaven and reaches up to the ears of God.
Martin Luther (AE 26:380)