In this verse, it is revealed that God communicates to our spirits that we are His children through His Spirit. Since we are in Christ, we are members of God's family, and His Spirit is constantly communicating with us. In some deep and unspoken way, He confirms to us what we already know to be true. Our confidence is directly related to the presence of God's Spirit in our lives.
In Christ, we abide, and in Him, we fellowship with the Father. It is His Holy Spirit who bears witness to the fact that we are God's children with our human spirit. We are blessed to have the Spirit of Christ as our regenerated spirit, giving us such beautiful reassurance. Despite His distance and aloofness, He is deeply involved in every aspect of our lives. We receive His assistance in every aspect of our lives in such a personal and intimate manner.
Having His presence in our hearts is not dependent on any outward signs, visible evidence, or audible evidence. Throughout the Scriptures, He promises to work His saving grace in our lives. As children of God, we are also infused with the Spirit of Christ. He is a sure witness to our hearts.

Dear Heavenly Father, it is You who have indwelled my human spirit and witnessed to my heart. Thank You for your Spirit, for confirming to me that I am your child and that I will enjoy all of your blessings. I want my life to be centered around You so that Your Holy Spirit can freely teach and guide me into all truth. Please continue to pour out Your grace more and more. Help me be faithful and character-driven under the weight of my trials and temptations. 
In Jesus's name I pray. Amen.