For the Holy Spirit speaks to us and tells our spirit that we are children of God.
Romans 8:16 (NLV)
When one comes to repentance and recognizes Jesus as his personal savior, he immediately receives the Holy Spirit which not only is the seal of his salvation, but also testifies to his spirit that he is now part of the family of God . However, this repentance must be true and sincere, and not a form of repentance to receive salvation.
By saying that the Holy Spirit speaks or testifies with our spirit, Paul indicates that the ministry of the Holy Spirit and that of Jesus are closely related. One makes salvation accessible to all, the other testifies to all those who accept it that they are children of God. Because of the sinful nature of men, the process begins in the person's heart. He sees the difference in his desire to submit to God rather than the lust of his flesh.
You might wonder how is it possible that the Holy Spirit bears witness to my spirit. The first thing you need to remember is that while our soul is connected to our intellect and our decisions, our body to what it can sense in the physical, our spirit is what leads us to need God. It is through our spirit that we relate to God. If you ever feel a spiritual void, it is because your sinful nature is in control. The Holy Spirit will always be there to remind you of who you are in Jesus Christ.
May the grace and the peace of God be with you all.