Jude is exhorting believers to contend earnestly for the faith which was once for all delivered to the saints (vs. 3) because certain ungodly men who have crept in unnoticed. Jude further describes these ungodly men as ones who cause divisions, worldly-minded, devoid of the Spirit.
The devil, 2000 yrs ago, created this false idea that if you teach the Grace of God, correctly, you are teaching "licence to sin".
Paul, himself, was accused of this very thing because He taught that the Grace of God, is an eternal mercy that deliveres you from judgment for sin, in eternity..= based on the "one time eternal sacrifice of Christ on the Cross for the sin of the World".
The Devil does not want this righteous truth preached and taught correctly, so, he subverted it, and twisted it, into the idea that this Grace of God, is "Licence to sin". And when a person gets that wrong idea in their head, as taught by a HERETIC, it become a spiritual and mental stronghold, that keeps them from being able to understand that God's Grace is not their self effort.
They become lost in trying to perform for God to TRY to be accepted, vs, resting in the Grace of God that is their eternal salvation provided as "the GIFT of Salvation".