"But whoso keepeth His word, in him verily is the love of God perfected: hereby know we that we are in Him" (I John 2:5).
The New Testament is emphatically clear that we are saved entirely by the grace of God through faith in Christ. "For by grace are ye saved through faith; . . . it is the gift of God" (Ephesians 2:8).
But how can we know for sure that our professed faith in Christ is genuine and we are really saved? Many who claim to be Christians are not truly saved, for Christ said: "Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven" (Matthew 7:21).
Now, note that John's main purpose in writing his gospel was to win people to saving faith in Christ. "These are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through His name" (John 20:31). Then the ultimate purpose of his first epistle was to assure them they were saved. "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" (I John 5:13).
His epistle, in fact, gives us several tests to prove our faith. One is in our text--we keep (literally "guard against loss") His Word. Also: "Hereby we do know that we know Him, if we keep His commandments" (I John 2:3). Then: "We know that we have passed from death unto life, because we love the brethren" (I John 3:14).
Thus, we can not only have salvation, but also assurance of salvation if we love and guard His Word, seek to keep His commandments, and love all others of like precious faith. Finally we have the indwelling witness of the Spirit. "Hereby we know that He abideth in us, by the Spirit which He hath given us" (I John 3:24). HMM