“Living in agreement with the devil” is an ambiguous term.
Only to those that don't understand what the Bible means when talking about walking after the flesh.
God is not mocked, if we walk after the flesh we shall of the flesh reap corruption (Gal 6) because sin in the flesh has been condemned (Rom 8)
There is scripture that speaks of believers falling away in the end times. One cannot fall away from something one had not ever received in the first place.
1 Timothy 4:1,2 - Now the Spirit speaks expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils; Speaking lies in hypocrisy; having their conscience seared with a hot iron
2 Timothy 4:3,4 - For the time will come when they will not endure sound doctrine (teaching); but after their own lusts shall they heap to themselves teachers, having itching ears; And they shall turn away their ears from the truth, and shall be turned unto fables.
2 Peter 2:1-3 - But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying (contradicting, Not agreeing with) the Lord that bought them, and bring upon themselves swift destruction.
And many shall follow their pernicious ways; by reason of whom the way of truth shall be evil spoken of.
And through covetousness (idolatry) shall they with feigned words make merchandise of you: whose judgment now of a long time lingers not, and their damnation slumbers not.
Jude 1:4 - For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness (no self control, living after the flesh), and denying (contradicting, opposing Jesus’ teachings) the only Lord God, and our Lord Jesus Christ. (referring to false teachers and false brethren - Matthew 13:24-30)
I’m not “ reformed” although I believe they are right about most issues.
Then you are reformed meaning you are friendly with false doctrine.