Read Galatians 4:4-7
4 But when the time had fully come, God sent his Son, born of a woman, born under law, 5 to redeem those under law, that we might receive the full rights of sons. 6 Because you are sons, God sent the Spirit of his Son into our hearts, the Spirit who calls out, “Abba, Father.” 7 So you are no longer a slave, but a son; and since you are a son, God has made you also an heir.
Paul's letter to the Galatians is to convince the Galatians, new, gentile believers, that they really belong to God's family.
Spirit testify in my heart that I am your child. Wipe away all my doubts.