Satan’s dark suggestion to the believer’s soul that God wishes him to remain always in doubt about his eternal destiny is a lie, for the Lord delights in the confidence and boldness of his people. Those who trust in the Lord should also be certain that God has given them eternal life, for faith is the condition of salvation which the Lord will always honour.
The verse also implies the possibility of the true believer lacking assurance, so that those who are struggling with it should not despair or conclude that they are disqualified and therefore should give up. Assurance is a natural part of faith, but is not so much of the essence of faith that those who struggle with it should conclude they are outcasts, otherwise John would not offer such reassurance.
From our assurance comes a greater freedom to praise God, to make great requests to him, to be fully engaged in his service, to resist temptation, and to handle the trials of life.