What does it mean to be truly a child of God? How can you be sure?
The answer has to do with what makes someone a Christian.
Is it going to church? Going to church is a great thing to do, but just going to church doesn't make you a Christian.
Is it telling the truth and trying to do good things? Telling the truth and doing good things are great things to do, but they don't make you a Christian.
The Bible says as many as received Jesus Christ, to them God gave the right to become children of God, even to those who believe on His name.
What makes you a Christian is to receive Jesus Christ as your Savior and Lord. We are to believe in Jesus and receive Him into our lives, asking forgiveness for our sins. That is what makes you a Christian, a child of God. If you have done this you have complete assurance you are a Christian. All the good works that you do are then done out of love and gratitude to God, but they don't earn your way into heaven. "He whom the Son has set free is free indeed."