There are two ways the Holy Spirit works by his witness to give you assurance that you’re really saved.
1. He leads you. In the context, we saw what the leadership is: leading you into hatred of and battle against your sin. “I hate it. I’m killing it. I’m a murderer. I hate my sin more than I hate anybody or anything.”
Do you do that? Do you hate your sin more than you hate the devil? Your sin’s a way bigger problem than the devil is. The devil cannot put you in hell. Your sin can. Your sin is the biggest problem in the world. Do you hate it? Do you make war on it? If you do, the Holy Spirit is loudly witnessing, bearing testimony: “You’re mine!” Because everybody that is led by the Holy Spirit is the son of God.
2. Do you, like a little baby, helpless, putting on no airs, say, “Abba, Father, I need you”? I think that’s what’s implied.
I think that’s why Paul said Abba. Why did he say “Abba” instead of just Father? Why didn’t he just say, “Everyone who says Father is experiencing the witness of the Holy Spirit”? Because he was trying to say, no, no, the real witness of the Holy Spirit is the work in the heart of a childlikeness. “Unless you turn and become like children, you will never enter the kingdom of heaven” (Matthew 18:3).
He has worked a childishness in you, a little, desperate, helpless, “I need help. I’m a nobody. I can’t save myself. I’ve got to have a daddy. I can’t live without help.” That’s the Holy Spirit talking. It is not a computerized, “Abba Father.” It’s his real, childlike, happy cry to the Father.
So, if you’re hating your sin and putting it to death, and if you are a brokenhearted, little child calling out to your Father, you’re a Christian. You can know it because he’s working those things in you.