3:20 us: 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.