The Greek word apeitheia is rendered, strikingly, as both unbelief and as disobedience. And when you consider the context of these passages, it is noteworthy that either translation works about as well.
In Romans 11, Pauls states it this way: “For the gifts and calling of God are without repentance. For as ye in times past have not believed God, yet have now obtained mercy through their unbelief: Even so have these also now not believed, that through your mercy they also may obtain mercy. For God hath concluded them all in unbelief, that he might have mercy upon all” (Rom. 11:29-32).
The same usage is found in Hebrews. “Seeing therefore it remaineth that some must enter therein, and they to whom it was first preached entered not in because of unbelief” (Heb. 4:6). “Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief” (Heb. 4:11).
But in Ephesians and Colossians, the same word is rendered as disobedience. “And you hath he quickened, who were dead in trespasses and sins; Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience” (Eph. 2:1-2). “For this ye know, that no whoremonger, nor unclean person, nor covetous man, who is an idolater, hath any inheritance in the kingdom of Christ and of God. Let no man deceive you with vain words: for because of these things cometh the wrath of God upon the children of disobedience: (Eph. 5:6). In a parallel passage, we see this: “Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry: For which things’ sake the wrath of God cometh on the children of disobedience” (Col. 3:5-6).
We could show from other places in Scripture that unbelief leads to disobedience, and that disobedience leads to unbelief. All this is true. But we should also see here that unbelief is disobedience, and that disobedience is unbelief.