The NT contains ample statements that people would/will be punished for unbelief. The corollary of a just God punishing people for X is that X is immoral.
Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God.
Whoever believes and is baptized will be saved, but whoever does not believe will be condemned.
But as for the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death.”
Then we have Paul saying that someone who doesn’t take care of his family is worse than an unbeliever. The obvious implication of this is that it’s immoral to be an unbeliever. The statement that a certain type of a person is worse than an unbeliever wouldn’t have rhetorical force if unbelief wasn’t immoral.