"For if those who live by law are heirs, faith has no value and the promise is worthless..." (Romans 4:14)
The quickest way to make the Word of God of non effect in your life is to rely on your own goodness as a means to achieve or obtain anything from God. The Good news of the Gospel is that everything has already been done by Jesus. Jesus was the promise to Abraham, because he was the Seed that the promise referred too. ( Galatians 3:16 )
So the Law introduced four hundred and thirty years later in no way voided out the promise to Abraham. God in his faithfulness sent Jesus at the appointed time, according to the promise, to redeem mankind legally from the dominion of sin and Satan. Therefore, the fulfillment of the promise in our own lives, by faith is what translates us into the kingdom of God.
The Law was also given for an appointed time. But Paul stresses in Galatians 3:21, that the Law was never able to impart life. However, Jesus said in John 10, "I have come that they may have life, and have it to the full." ( John 10:10 ). So clearly, the Law brings death and the promise brings life.
If we live according to the dictates of the Law then it nullifies our faith. Because our faith is misappropriated in the works of the Law. We choose to rely on our good deeds or we choose to rely on Jesus. It cannot be a combination of both. The promise through Jesus is what brings about our justification in the sight of God.