For you did not receive a spirit that makes you a slave again to fear, but you received the Spirit of sonship. The Spirit himself testifies with our spirit that we are God's children. It is no accident that Jesus portrays his disciples receiving eternal life because they hear his voice and follow him. This is the life of a true believer. Assurance and confidence comes as we express our confidence in Christ commands through obedience.
Understanding is the reward of faith.
Therefore, seek not to understand that thou mayest believe, but believe that thou mayest understand. John gives us several birthmarks so that we can be confident that we have been born of God. If you know that he is righteous, you know that everyone who does what is right has been born of him. We know that we have passed from death to life, because we love our brothers. Anyone who does not love remains in death.