Can a Christian renounce Christ to follow the devil?

Jesus describes the process of salvation as being "born again" in John 3 because salvation requires a spiritual rebirth. Literally speaking, our spirit must be brought back to life by the power of the Holy Spirit, and in that way, we are made spiritually new. As Paul explains:

Eph. 2:1  And you were dead in your trespasses and sins,
Eph. 2:2 in which you formerly walked according to the course of this world, according to the prince of the power of the air, of the spirit that is now working in the sons of disobedience.
Eph. 2:3 Among them we too all formerly lived in the lusts of our flesh, indulging the desires of the flesh and of the mind, and were by nature children of wrath, even as the rest.
Eph. 2:4 But God, being rich in mercy, because of His great love with which He loved us,
Eph. 2:5 even when we were dead in our transgressions, made us alive together with Christ (by grace you have been saved),
Eph. 2:6 and raised us up with Him, and seated us with Him in the heavenly places in Christ Jesus,
Eph. 2:7 so that in the ages to come He might show the surpassing riches of His grace in kindness toward us in Christ Jesus.

Paul is describing being born again. Before coming to faith in Jesus, we were physically alive, but spiritually we were "dead in our trespasses" and by nature children of God's wrath. That state had to be corrected before we could receive eternal life and be welcomed into the presence of the Father. So the Spirit of God must enter our heart and cause us to be born again, which leads us to cry out to God and confess Christ, as Paul explains:

Gal. 4:6 Because you are sons, God has sent forth the Spirit of His Son into our hearts, crying, “Abba! Father!”
Gal. 4:7 Therefore you are no longer a slave, but a son; and if a son, then an heir through God.

Therefore, because being born again is a work done by God in our Spirit, our salvation cannot be reversed. We cannot be "unborn" spiritually any more than we can be "unborn" physically. As Paul says:

2Cor. 5:17 Therefore if anyone is in Christ, he is a new creature; the old things passed away; behold, new things have come.

So once a person has believed in Jesus Christ and has been born again by the Spirit, that person is forever a child of God. Paul says we are adopted into the family of God:

Eph. 1:3  Blessed be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly places in Christ,
Eph. 1:4 just as He chose us in Him before the foundation of the world, that we would be holy and blameless before Him. In love
Eph. 1:5 He predestined us to adoption as sons through Jesus Christ to Himself, according to the kind intention of His will,
Eph. 1:6 to the praise of the glory of His grace, which He freely bestowed on us in the Beloved.

So then what do we conclude about a person who renounces their faith in Jesus or claims to follow the devil, etc.? Knowing it's impossible for someone born again to reverse that process, then there are the only two possibilities: either the person was never truly born again by the Spirit and has now reverted to their true nature (i.e., a child of wrath), or the person was saved and remains so though they are rebellious and disobedient and will likely receive discipline from God (see Hebrews 12).

Remember, Christians can and do disobey Christ even after being saved. Christians can even go so far as to rebel against Christ to the point of denying Him. Even the Apostle Peter denied Christ three times! Therefore, we cannot assume that egregious sin – even a denial of Christ – means a person was not saved in the beginning.

By the same token, we cannot assume that every confession of Christ is sincere. Typically, false confessors will abandon their charade after a time and renounce their "faith" – though it was a faith they never truly held in the first place. 

The Lord knows who are truly His, and the judgment day will reveal it to everyone. For a complete understanding of matters related to salvation, please listen to our Romans Bible study.

Scripture quotations taken from the (NASB®) New American Standard Bible®, Copyright © 1995, 2020 by The Lockman Foundation. Used by permission. All rights reserved. www.lockman.org