The apostle John's vocabulary in his gospel, epistles, and even in Revelation is quite distinctive. The verb "know," for example, occurs more in John than in any other gospel, and more in 1 John than in any other epistle. He emphasizes by this that the Christian life is based on knowledge. In the words of today's verse, for example, we can test the genuineness of our knowledge of Christ as Savior by whether or not we keep His commandments. Note some of the other tests listed in John in his first epistle, as follows:
"Ye know that every one that doeth righteousness is born of him" (1 John 2:29). "We know that we have passed from death unto life, because we love the brethren" (3:14). "Hereby we know that he abideth in us, by the Spirit which he hath given us" (3:24). "But whoso keepeth |i.e., 'guards'| his word, in him verily is the love of God perfected: hereby know we that we are in him" (2:5). "These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life" (5:13).
There are other similar "tests of life," but these make the point. A person who has been really born again through faith in Christ and His saving work can have assurance of his salvation, if he truly believes in the name of the Lord Jesus Christ; if he guards and honors God's Word; if he manifests the presence of the guiding, purifying Holy Spirit in his life; if he keeps His commandments and lives righteously, and if he manifests real love for his Christian brethren.
This is not to say that if he fails one or more of these tests he is necessarily unsaved. There are, however, no grounds for real assurance of salvation without them. Therefore, as Paul suggests, "Examine yourselves, whether ye be in the faith; prove your own selves" (2 Corinthians 13:5). HMM