Biblical support for why something is a sin?

The professor of russian Orthodox Church Osipov defines sin not as law given by God to human, but as advice. Human, don't do this, this will cause you to bleed. Do not harm yourself. So your question is actually about why this or that thing is harmful.

So if you think about love to enemies, holding off your own wishes and other words of Christ, you definitely will see that fulfilling them brings joy for human and disobeying them causes him to suffer.

There are several related topics here. I'll address each one separately.

"I want to understand!"

let me know Your ways that I may know You - Exodus 33:13

First, I want to emphasize that it is good to seek understanding. It is also good to help others understand His ways. Seeking to know the reason behind a command is not sin.

Philip ran up and heard [the Ethiopian] reading Isaiah the prophet, and said, “Do you understand what you are reading?” And he said, “Well, how could I, unless someone guides me?” And he invited Philip to come up and sit with him... Then Philip opened his mouth, and beginning from this Scripture he preached Jesus to him. - Acts 8:30-35

Knowledge of scripture will only take you so far; at some point you need to understand what you are reading - and often times you need someone to be there to help you understand. If Philip had not explained the Scriptures to the Ethiopian, he would not have understood and would not have been converted. We should be careful how we respond to a person who is seeking to understand God's ways.

General Justification

Your hunch is correct - all sin in Scripture does indeed have some degree of justification, although it may not be specific or explicit. I tell my kids not to eat batteries... because if they do they will die. Similarly, God tells us not to sin... because if we do we will die.

God is love. Believe it or not, all of His commandments were given to teach us love - and to show us our inherent inability to love - so we would return to God through Christ! His commandments are an elaborate attempt at urging us to return to Him! The law was given to teach us about God's very nature - that which is good and right.

Sin is a rejection of what is good and right; a rejection of God's law; a rejection of His ways; a rejection of Him. Sin separates us from God. If we choose sin, we choose separation from God - He cannot "embrace separation", "accept rejection", "fellowship with those who flee Him"... If we die in this state, we experience eternal separation from Him, also referred to as "torment" and "death".

The general justification of every command is the same. It is good to follow God. It is bad to reject God. Sin creates a separation between you and God. This is really bad for you, so you should forsake sin and return to Him.

No Explicit Justification

There are examples of commands in the Bible which have no explicit justification. Here's one random example:

They shall construct an ark of acacia wood two and a half cubits long, and one and a half cubits wide, and one and a half cubits high. You shall overlay it with pure gold, inside and out you shall overlay it, and you shall make a gold molding around it. - Exodus 25:10-11

Why those specific dimensions? Why acacia wood? I don't know. I am sure there was a good reason - and perhaps there is a Bible scholar out there who can explain it using Scripture. But to my knowledge no explicit justification is given in Scripture.

Does God Explain Himself?

• God invites His people to seek understanding. He is in the business of explaining things to us. Indeed, He promises to lead us into the truth we seek.

• However, God has no interest in granting understanding to scoffers. If a person wants to judge God, they will be met with rebuke, not explanations.

Proper Handling

Your question here was, "If there is a command without scriptural justification, what is the right course of action in defending the command?" First - be cautious about entering into the activity of teaching others about the Word of God.

Let not many of you become teachers, my brethren, knowing that as such we will incur a stricter judgment. - James 3:1

If you are called to teach, make sure you know what you're talking about before you begin making confident assertions about things...

For some men, straying from these things, have turned aside to fruitless discussion, wanting to be teachers of the Law, even though they do not understand either what they are saying or the matters about which they make confident assertions. - 1 Timothy 1:6-8

If God has gifted you to teach, and given you understanding about the topic you are discussing, the proper method of teaching is summed up pretty well here:

The Lord’s bond-servant must not be quarrelsome, but be kind to all, able to teach, patient when wronged, with gentleness correcting those who are in opposition, if perhaps God may grant them repentance leading to the knowledge of the truth, and they may come to their senses and escape from the snare of the devil, having been held captive by him to do his will. - 2 Timothy 2:24-26

The teacher must also be led by the Spirit and should function within the context of the other members of the body of Christ.

This is the most formulaic approach I can concisely offer for how to properly defend a command from scripture. I hope that helps!

What is Sin?

Before we ask, "why is something sin?", we must ask "what is sin?". The Bible doesn't give an explicit and all encompassing definition of sin. Based on what has been revealed to us about sin, the Catholic church defines sin as follows -

"Sin is an offense against reason, truth, and right conscience; it is failure in genuine love for God and neighbor caused by a perverse attachment to certain goods. It wounds the nature of man and injures human solidarity. It has been defined as "an utterance, a deed, or a desire contrary to the eternal law. Sin is an offense against God: "Against you, you alone, have I sinned, and done that which is evil in your sight... (Psalm 51:4)" (CCC 1849-1850)

Why is something evil in God's eyes? (i.e. why is something sin?)

God is the objective standard of morality. Without God, there is simply no objective morality as Ravi Zacharias explains in this short video.

Before going to God, let's ask, "why is something evil in my eyes?" I answer, "It is evil because I KNOW it is evil. It is evil because it contradicts what I know is good and right." I don't need God to tell me that murder is evil. I know within my heart and mind that it is evil, because my conscience (Romans 2:14-15) tells me so." Where does my conscience come from? My conscience comes from God because God created me and gave me a conscience. However, because of the fall of man, my conscience is distorted and therefore, my conscience cannot always tell me whether something is objectively evil or not. See Romans 14:14 for an example of this.

In the same way, something is evil in God's eyes because God KNOWS what is good and evil (Gen 3:5). And he knows it perfectly. Since God, by definition, is the Supreme Good itself, anything that goes against God's will or against his nature is evil and God knows it. So, for eg. sexual relations between a man and an animal is evil, because God didn't intend it to be so. God knows it is not good for man. But man may not always know this, because of his fallen nature.

In the ultimate analysis, as David Morton said, we have to trust God, because he is the objective standard of morality. If God says it is evil, then it must be evil, because he is the only standard of objective good and evil. He knows what is good and evil; we may not always know.

Why Sin is Sin

Sin is not sin because the Bible says so. Sin is sin because it is contrary to the nature of God, it separates mankind from God, and it is self-destructive. Indeed, the thief comes to steal, kill, and destroy.

So, sin is not sin just because the Bible says so. Sin is sin because of what it is. The role of the Bible is to identify for mankind what sin is and what it is not.

The Moral Compass

It seems that most all of us accept the idea of right and wrong, good and evil. In many cases, there is widespread agreement, but there certainly is disagreement in significant areas as well. However, while most everyone would agree that murder is wrong, many in Germany in the 1940's believed it was not wrong to murder Jews. In fact, there are many today who still believe it is morally right to murder people of other faiths or those who convert to other faiths.

The question, then, is how can humanity discern who is right and who is wrong about what is right and what is wrong? We can't vote on it, because a majority who believe genocide is alright are still wrong. That leaves us with really only one solution, and that is to allow the Moral Lawgiver to define morality. If He says it's wrong, it's wrong--despite what we believe.

If He says adultery is wrong, then there is no valid excuse an adulterer can give. If He says it's wrong to blaspheme His Name, then the blasphemer has no excuse. If He says it's wrong to lie, then politicians are all in trouble.

So, the Word of God is the moral compass, and we must allow God to be the final word to moral issues.

Two Responses

That being said, most "sins" are pretty self-evident. Most people know inherently that lying, cheating, stealing, rape and murder are all wrong.

The problem comes with things that are disputed. For this, we have the option of either accepting or rejecting God's input on the matter. If we read the Bible and we disagree with its position, there are two responses we can have.

One person sees the disagreement and concludes that God is wrong. This person believes that he himself knows better than God or discounts the relevance, accuracy, or authority of the Word of God. Such a person, by definition, is a follower his own way.

Another person sees the disagreement in the Bible with his life or ideas and concludes that he himself must be wrong. Such a person, by definition, is a follower of the Bible and of the God of the Bible.