Christians are sons & daughters of God

1 John 4:15-19 Whoever confesses that Jesus is the Son of God, God abides in him, and he in God. And we have known and believed the love that God has for us. God is love, and he who abides in love abides in God, and God in him.
Love has been perfected among us in this: that we may have boldness in the day of judgment; because as He is (Jesus is the Son of God), so are we in this world. There is no fear in love; but perfect love casts out fear, because fear involves torment. But he who fears has not been made perfect in love. We love Him because He first loved us.
We are not "The" Son of God, but in Romans 8:29 it does say Jesus was the Firstborn among many brethren. When we are born again we literally become a son or a daughter of God. We can say, "God I love You" because He loves us first. God literally becomes our Father and now when He says, "I love you", from His perspective He is saying, "I love you as My son or My daughter." This is the flawless (perfect) love that casts out all fear. God our Father loves us like a daughter or son. Let your Father reveal to you how much He loves you. Over the next several days ask Him, "Father, please tell me and show me how much You love me."