Be careful to live properly among your unbelieving neighbors. Then even if they accuse you of doing wrong, they will see your honorable behavior, and they will give honor to God when he judges the world~ 1^st Peter 2:12 NLT 

The word heresy is just a fancy-pants theological term for the act of departing from a pattern of sound biblical teaching (1^st Timothy 6:20-21, 2^nd Timothy 1:13, 2^nd Timothy 4:3). 

Sometimes heresy takes the form of some seriously insane theories about God. 

 At one point there was a fairly large group of Christians who believed that Jesus was just a human being who was formally adopted by God at his conception. Once the adoption was “finalized” he developed a divine (God) nature while growing in Mary’s body (Adoptionism). Other early believers were convinced Jesus was a phantom who didn’t leave footprints when He walked rather than a flesh and blood person (Gnosticism). For nearly a thousand years some “Christians” believed people are born without a sin nature and are capable of living a holy life apart from Jesus and the regeneration of the Holy Spirit (Pelagianism).  

Most heresies are not crazy-pants lies about God or obvious misinterpretations of Scripture. Most heresies are more like tiny kernels of actual theological truth wrapped-up in a bunch of half-truths and strange errors.  Two such heresies are deeply rooted in our contemporary Christian culture. The first is legalism. Legalists believe they can earn God’s favor by doing all the right things and obeying all the right rules. 

God really does care about our behavior (more on that later). However, even the best-behaved person in the world cannot save themselves from their own sin (Ephesians 2:8, 2^nd Timothy 1:9, Hebrews 10:39). Legalism sidetracks Christians from relationship with Christ by placing the emphasis on what we can do for ourselves rather on what Jesus did for us. This eliminates the element of of worship and gratitude from Christianity. Legalism also falsely paints God as demanding, callous and impossible to please. This leads some legalists to feel discouraged and resentful towards God. This can lead to hopelessness and eventually even a departure from the faith.

On the other end of the doctrinal spectrum is licentiousness or the believe that there are no rules for Christians. These folks believe once a person is saved there is nothing they can do or not do to offend God or break relationship with God. Christians who have intentionally or unintentionally adopted this view do not worry seriously about the effects of sin, even deliberate, premeditated sin on their relationship with God. Adherents to this view are growing in number and having an enormous impact on the greater Christian culture. 

The truth is that our behavior does matter, not because good behavior saves us, or because “being good” makes God like us better. Correct behavior and following the rules matter for four reasons:

Righteous behavior protects us from moral failure and the pain that accompanies moral failure– 

Ephesians 6:14 instructs Christians in a metaphorical sense to put on the “breastplate of righteousness”.  The primary purpose of a breastplate in Roman body armor was to protect the soldier’s heart from injury. In Proverbs 4:23 the writer instructs readers to “guard your heart because everything you do flows from it”. Behaving in a way that is righteous (avoiding sin and questionable behavior) protects us from all sorts of pitfalls, wrong thinking and potential moral disasters. For example, going out of your way to avoid pornography protects against addiction, the sin of lust and at least a dozen other really ugly sins. Avoiding those who gossip ensures that you will not become a slanderer (Psalm 15:1-3).  

When Christians behave virtuously non-Christians have the opportunity to experience something the Bible calls conviction-

Perhaps the most critical reason to avoid sin and to behave righteously is because when we do the people around us have a model of good behavior to follow. Sometimes our good behavior even leads sinners to feel guilt or conviction over their bad behavior (1st Peter 3:13-16). Conviction often leads to repentance. Repentance leads to salvation. Being a part of someone else’s salvation experience is a huge blessing in every sense.

Christians are commanded to avoid certain behaviors and sins-

The New Testament gives a series of “sin lists” addressed to Christians (Mark 7:21-22, 1^st Corinthians 5:10-11, 1^st Corinthians 6:9-11, 1^st Timothy 1:8-11, Colossians 3:5-8, Galatians 5:19-21). Most of time those lists are predicated with or followed by the caution that people who routinely practice the sins listed will not “inherit the kingdom of God”.  In my view it is reasonable to question the salvation of any “Christian” who chooses not to take those warnings to heart. 

Bad behavior causes Christians to lose their moral authority-  

There was a time in the not-so-distant past when most people (saved or unsaved) looked to the church for moral direction and spiritual guidance. However, allegations of fraud, sex abuse scandals, infidelity and wholesale hypocrisy amongst clergy and laypeople alike have stripped the church and its people of any moral authority we once had. Now our culture is swimming in moral chaos and thanks to the sinful antics of Christians over the last forty years no one is looking to the one source that truly has the answers to our problems: the church. 

God loves humanity so much that He sent His Son, Jesus, to save people from their sin (John 3:16). His love doesn’t end there though. God also loved us enough to give us moral boundaries to keep us from going off the rails after we come to know Jesus. When we don’t stay within the boundaries God gave us we create disaster for ourselves and run the risk of leading others astray.