To translate this 'sin is the transgression of the law,' as in A.V., is wrong, and gives a false definition of sin, for sin was in the world, and death as a consequence, before the giving of the law: see Rom. 5.13; 7.13. The Greek reads 'sin is lawlessness,' that is, the absence of the principle of law (not the law), or, in other words, of the control of God over the soul. I ought to have no will of my own, but be in obedience. The statement is reciprocal, and may be read 'lawlessness is sin.'
Subjunctive. 'That,' ver. 20, refers to 'hereby' in ver. 19. It is showing what tests 'in truth.' The repetition of 'that' is nothing uncommon. It is found in Eph. 2.11,12. What 'hereby' refers to very commonly follows in John, as in vers. 16,24, and ch. 4.17, and elsewhere, frequently indeed with 'that.' 'God being greater' is evidently a testing, searching thing.