At the end of act 3, Danforth has John Proctor arrested for colluding with the devil, and Proctor announces that God is dead. He is completely overwhelmed and furious with the proceedings. Proctor then displays his complex, guilty conscience by lumping himself in with Deputy Governor Danforth and saying that they share the same filthy face as Lucifer.
He goes on to mention that he has cringed out of fear and shame in the same way that Danforth quails because he knows in his black heart that the girls are frauds. He then says that God damns their kind and believes that they will surely burn together in hell. Even though Proctor is clearly a more morally-upright man than Danforth and has sinned to a lesser degree, he insists that his sins are equal to Danforth and compares his guilty conscience to Danforth's tainted soul.
Proctor believes that Danforth knows in his heart that the girls are frauds and secretly quails when he thinks about the destruction he has caused. Proctor's comments that he and Danforth both share the devil's features is also significant and suggests that every individual has a capacity for evil. John has also reached a tipping point, where he fully accepts responsibility for his sins and is willing to embrace the consequences. He is also adamant to expose Danforth as well by revealing that he has quailed over the course of the...
Overall, John Proctor's comment reflects his guilty conscience and awareness that Danforth is also a sinner who quails when he thinks about his sins but remains resolute in public to save face. Proctor also believes that his sin of adultery will result in an eternity in hell, which is the same punishment Danforth will receive.