From the moment you turn from your sin to Jesus, God begins a wonderful transformation of your life (2 Cor. 5:17). Every new believer is given a new spiritual heart and God's Holy Spirit to guide him. As wonderful as this change is, there are times when you may experience doubt about what has happened. Does God really love me? Did he really forgive me of my sins? At times like this, the best thing to do is follow the advice of Scripture.
For this very reason, make every effort to supplement your faith with virtue, and virtue with knowledge, and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, and godliness with brotherly affection, and brotherly affection with love. For if these qualities are yours and are increasing, they keep you from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ. For whoever lacks these qualities is so nearsighted that he is blind, having forgotten that he was cleansed from his former sins.
—2 Peter 1:5-9
In these verses, Peter tells new believers that assurance comes from a growing relationship with God. As you continue believing God's promises, add to your faith virtue, knowledge, self-control, perseverance, godliness, brotherly kindness, and love. These characteristics are developed with the help of the Holy Spirit as you submit to his leadership every day. Then, as these characteristics become a daily habit in your life, God will give you assurance that you are his child and will keep you from stumbling back into your former sinful life.