When, as Christians, we go through times of deep despair, depression, loneliness, pain or suffering, we instinctively just cry out ‘Father’. This cry is itself a sign that we are sons of the living God. In our times of great need, the Spirit ministers to us and enables us to cry out ‘Abba, Father’.
When we do, this is the hallmark of a believer. This is the cry of the child of God.