See “for obedience to Jesus Christ,” 1:2; “obedience to the truth [of the gospel],” 1:22; “they disobey the word [of the gospel],” 2:8; “some do not obey the word [of the gospel],” 3:1; “those who do not obey the gospel of God,” 4:17; cf. outside of 1 Peter — 2 Thes 2:8; Rom 1:5; 15:18; 16:26; Acts 6:7.
The gospel message is not merely something to be believed; but, rather, it is also something to be obeyed. It is not merely a proposition to be heard and considered — something like, “Believe it if you feel like it.” No. Accompanying the gospel is a summons straight from God himself, a summons to believe. To not believe is not to take a neutral stance towards the gospel. It is to rebel; it is to disobey God himself, rejecting his very appointed means of salvation (see 1 Peter 2:4-8).