The book of 1 John was written to give assurance to true believers (1 John 5:33). What does it tell you is the way to know that you are saved? There are many statements in the book which answer that question. Although each of these may put it differently, 1 John 2:29 is as clear and concise as any of them:
Since you know that He is righteous, you may know also that whoever is doing righteousness has been born of Him.
Assurance comes from a change in one’s life that John says is a birth which God brings about: “whoever is born of Him.”
This new life is the result, John says, of what we call “regeneration” (being given spiritual life that enables one to believe the Gospel). To be born of God also enables him to live differently—to live righteously. Apart from this spiritual birth one is unable to do those righteous things that God approves: “Those who are in the flesh cannot please God” (Romans 5:5). To be “in the flesh” means to have only the natural, sinful life with which you were born—to be unregenerate (that is, to be without the newness of life that comes only with being born of God). So, those who do righteousness may know that they have been born from above.
The interesting phrase that is central in the verse is “doing righteousness.” When one sees the fruit of the Holy Spirit (righteousness), Who regenerates, begin to appear in his life, he may know that the change has taken place—that he is a child of God? Notice, it is not when he feels something different, but when he does things differently, that he may have assurance of his salvation.
How can one know what these righteous acts are? By studying the Bible. There alone is the standard of righteousness.
Friend, do you do righteous things—those that God says please him? If your life has been changed so that more and more it conforms to that standard, you have reason to believe you are saved.