Now being in unbelief means a few things spiritually for all sinners naturally. Naturally the elect as well as the non elect are spiritually blind to the spiritual things of God Eph 4:18
Having the understanding darkened, being alienated from the life of God through the ignorance that is in them, because of the blindness of their heart:
2 Cor 4:4
In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, who is the image of God, should shine unto them.
Secondly, the elect by nature in unbelief have hardness of heart towards God and spiritual matters. In fact the word blindness in Eph 4:18 is the grreek word pōrōsisand means:
stupidity or callousness:—blindness, hardness.
Paul writing of jews by nature writes Rom 2:5
But after thy hardness and impenitent heart treasurest up unto thyself wrath against the day of wrath and revelation of the righteous judgment of God;
However that doesnt just apply to the unregenerate jews, but to unregenerate gentiles as well, who are in unbelief.
And thirdly to be in unbelief is to be spiritually alienated from the life of God Eph 4:18
Having the understanding darkened, being alienated from the life of God through the ignorance that is in them, because of the blindness of their heart:
Col 1:21
And you, that were sometime alienated and enemies in your mind by wicked works, yet now hath he reconciled
This is how God finds His elect for the purpose of mercy !