5:10 hath the witness. That internal witness is none other than the indwelling Holy Spirit, who is both “in heaven” (as the third person of the Godhead) and “in earth” (note I John 5:7-8), as He indwells each believer. Compare Romans 8:16-17; Galatians 4:6.
5:10 made him a liar. Those who recoil at the thought of eternal punishment of the lost need to reckon with the infinite magnitude of their sin—that of calling their own Creator a liar! An infinite sin warrants infinite punishment, especially in light of the infinite sacrifice made for them by their Creator and the free gift of infinite life (I John 5:11) offered them by that loving God on the basis of His sacrifice. The punishment is more than merited by the crime.
5:10 the record. The same Greek word, in either verb or noun form (martureo, marturia) is translated “record” three times in this epistle, “testify” twice, and “witness” seven times. These words are also very common in John’s other writings. The frequent cost of such a “witness” is pointed up by the fact that we derive our word “martyr” from this Greek word.