Simply put: defeat doubt by immersing your mind in the Word of God. This is the ordained means by which the Spirit will indelibly imprint on your heart the joyful and undeniable assurance that what God has said, God will do.
Paul said much the same thing when he prayed that the Romans might ‘abound in hope’ Romans 15:13b). But abundant hope or full assurance only comes ‘in believing’ (Romans 15:13a) or in connection with and as a result of our faith in what God has made known. Sin-killing, Satan-silencing confidence doesn’t fall from heaven like manna, nor do we serendipitously bump into it as we skip blissfully and ignorantly down the yellow brick road to a heavenly Oz. The Spirit imparts hope and confidence and assurance by means of and only in connection with our growth in the knowledge and understanding of God in his Word. ~ Sam Storms, The Hope of Glory