Hebrews 3:12 “Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.”
In this context, “an evil heart of unbelief” is a “heart” that has been sickened by carnal lusts and human philosophy and false doctrine; somewhat like being sickened by some devastating contagion. When religionists add to or exclude things from the Word of God, they are disbelieving God’s Word; the net effect is that they depart “from the living God”; that is, depart from His Doctrine and Commandments. Our Lord calls it “the commandments of men” (Mark 7:7) and “the tradition of men” (Mark 7:8). The Jewish Christians in Biblical times were subject to efforts to draw them back into the Old Sacrificial Works System devised by their leaders. The Apostle Paul explains the heresy with this question, “O foolish Galatians, who hath bewitched you, that ye should not obey the truth, before whose eyes Jesus Christ hath been evidently set forth, crucified among you?” (Galatians 3:1). In our day, Christians often succumb to various Works Systems that deny our Lord’s “finished” sacrificial work (John 19:30) which did indeed “save his people from their sins” (Matthew 1:21); that is, the Lord Who “hath saved us, and called us with an holy calling, not according to our works, but according to his own purpose and grace, which was given us in Christ Jesus before the world began,” (II Timothy 1:9). The point is, did Jesus Christ Sacrifice Himself to make His people savable or did He endure such an awful Sacrifice to “save his people from their sins”? This “unbelief” is “evil” because it is indeed like a devastating disease, it will ultimately rob us of all joy and peace, leaving us broken and discouraged. This is why ministers are Commanded to “Preach the word” (II Timothy 4:2); not some man’s “commandments” or “tradition”.