And the Holy Spirit also bears witness to us; for after saying, “This is the covenant that I will make with them after those days, declares the Lord: I will put my laws on their hearts, and write them on their minds,” then he adds, “I will remember their sins and their lawless deeds no more.”
The Spirit himself bears witness with our spirit that we are children of God,
these things God has revealed to us through the Spirit. For the Spirit searches everything, even the depths of God. For who knows a person’s thoughts except the spirit of that person, which is in him? So also no one comprehends the thoughts of God except the Spirit of God. Now we have received not the spirit of the world, but the Spirit who is from God, that we might understand the things freely given us by God. (ESV)