John’s declaration of the kingdom of God being at hand (Matt. 3:2) was also associated with apocalyptic realities, such as “the wrath to come” (v. 7), bad trees being axed and “thrown into the fire” (v. 10), and chaff being burned “with unquenchable fire” (v. 12). Both John and Jesus (cf. 4:17) were simply reiterating the heart of the prophetic and apocalyptic traditions: “The day of the Lord is at hand” (cf. Isa. 13:6; Joel 1:15; Zeph. 1:7; Mal. 4:5; 1 Enoch 1:1; 51:2; 96:8; 100:7; 4 Ezra 4:26; 6:18; 9:2; 2 Bar. 23:7; 31:5; 76:5; 82:2; 83:1). 

Because of the associated divine judgment, the proclamation of the kingdom of God had a negative edge designed to evoke repentance.  - John P. Harrigan