The main difference between Delusion and Denial is that a deluded person may never have been exposed to the truth while the person in Denial has but chooses not to accept it.
So, in effect, the person in Denial chooses Delusion.
The result is the same for both states - Bondage to a Lie.
Satan comes to Steal, Kill and Destroy. He wants to Steal the Truth which sets you Free so that you stay in Bondage to the Lie.
There is another difference between Denial and Delusion that should not be minimized. The person in Delusion does not carry doubt while the person in Denial does.
When a person's Conscience is seared they lose the Guilt so, in most cases, the person in Denial eventually ceases to have any Doubt and slides into total Delusion.
Affinity for the truth is important to us, not only Spiritually but also Psychologically.
We train our Minds to operate in a certain way. A compulsive Liar has trained his Mind to prefer Lying over Truth and it actually becomes easier for them to Lie than to speak the truth.
In the same fashion we can train our Minds to prefer Truth to the rejection of anything that resembles a Lie.
Jesus trained His Mind so that He could with confidence say that the Words He spoke were the Words of His Father. Joh 12:49
Giving space to a single Lie in your Mind will block the flow of God's Truth to you by the HolySpirit.
Denial in any area of their life is not an option for a Christian desiring to operate with the Mind of Christ.
So purge your Heart of Doubt (believing a Lie about God) to avoid having an EvilHeart. Heb 3:12