In 1 Timothy 4:2 Paul writes that those lying advocates of false religion have been “seared in their own conscience as with a branding iron”.  Such is a particularly vivid image likening them to slaves who like unreasoning animals (2 Pet 2:12, Jude 10) have been branded with a spiritual mark of ownership.  For the true sons of God who have been redeemed by the Lamb are called the bond-slaves of God (cf. Rom 1:1, Jam 1:1, 2 Pet 1:1, Jude 1:1, Rev 1:1) and as such receive a seal on their foreheads marking them with His name (Rev 7:3, 14:1, 22:4).  That seal on their foreheads is a tender conscience that has been regenerated by the word of God that is ever before it (cf. Ex 13:9-16, Dt 6:6-8, 11:18, Rom 12:2, Eph 4:23, Tit 3:5).  Like a sensitive moral compass it always points them to the Lord so that beholding His glory they are transformed into His image and come to reflect His likeness in their own countenance with ever increasing glory (2 Cor 3:18, cf. Ex 34:29, Ecc 8:1, Mat 13:43, 17:2, Acts 6:15, 2 Cor 3:7-8).  In this same way the false sons who have been deceived by that beast who has “two horns like a lamb” (Rev 13:11, cf. Mat 7:15) are also his slaves and receive a mark of his ownership on their foreheads (cf. Rev 13:16-17).  But as Paul notes here, that mark is a conscience that has been seared or cauterized so as to deaden it to all feeling.  For in their hearts they have no real love for the truth but love the world and the things in the world.  They never bind the sanctifying word of truth as frontals before their eyes to meditate upon it and be transformed by the renewing of their minds.  Nor do they bind it as a sign on their hands in order that they might be doers of the word and not merely hearers who delude themselves (cf. James 1:22-25).  Instead they keep ever before them the words of the false prophets (with Rev 13:11-18 confer Rev 16:13, 19:20, and 20:10) that give them a false security that they can continue in their stubborn rebellion against God and not surely die.  They suppose that so long as they have some token works of their hands to offer God as a sign of their religion they can continue to indulge their carnal nature.  In this way through unrepentant sin their consciences become seared and no longer serve as a moral compass to point them to the Lord.  Instead, beholding the form of godliness of the one they really serve they are transformed into his image—an image of the beast—and come to reflect his likeness in their own countenance with ever increasing shamelessness:

The expression of their faces bears witness against them.  And they display their sin like Sodom; They do not even conceal it.  Woe to them!
Isaiah 3:9

O LORD, do not Thine eyes look for truth?  Thou hast smitten them, but they did not weaken; Thou hast consumed them, but they refused to take correction.  They have made their faces harder than rock; they have refused to repent.
Jeremiah 5:3

Were they ashamed because of the abomination they have done?  They were not even ashamed at all; They did not even know how to blush.
Jeremiah 6:15

Therefore the showers [of God’s Spirit] have been withheld, and there has been no spring rain.  Yet you had a harlot’s forehead; you refused to be ashamed.
Jeremiah 3:3

“How languishing is your heart,” declares the Lord GOD, “while you do all these things, the actions of a bold-faced harlot.”
Ezekiel 16:30