St. Philaret of Chernigov: Lack of faith in Christ is the root of sin

Lack of belief in the Son, the Redeemer of the world, is the seed and root of sins, the essence of all transgressions of the will. Lack of faith in the Redeemer and Son includes within itself, as in a kernel, pride, carelessness of oneself, all fleshly indulgences, inattention to the dignity of others, and forgetfulness of God... As long as the sin of unbelief lies upon the soul, salvation is impossible for it. For what reason did Israel not enter into the promised land? For lack of faith, replies the Apostle¹ (cf Heb 3:19). Lack of faith in the Son of God is fearful. For there is none other name under heaven given among men, whereby we must be saved. (Acts 4:12) And it is this fear that the Spirit-Reprover arouses in the unbelieving world.
+ St. Philaret of Chernigov, from On the Passion of our Lord

Jesus Christ, homily 20.