Romans Chapter 8:15-16
For you did not receive a spirit of slavery to fall back into fear. Instead, you received the Spirit of adoption, by whom we cry out, “Abba, Father!” The Spirit himself testifies together with our spirit that we are God’s children

Under the Law you required 2 testimonies in order for something to be true. Here Paul tells us that we are children of God and we have 2 that testify to that fact, ourselves and the Holy Spirit. Together with the Holy Spirit we can cry out to God like a child crying out for their father, and we have nothing to fear when we are safely in the arms of our Father.