What is the Biblical basis for the concept of 'balance' between 'two extremes'?

This has been an interesting and enlightening read. I will just like to add to it by saying,

Whatever 'balance' that does not eventually lead to sin remains healthy for one's spiritual life...on the other hand, whatever 'balance' that threatens your spiritual nature in Christ should by all means be avoided.

Eg 'balance/moderation' will not work for someone who has an addiction weakness to say alcohol or sex. Extreme avoidance in such situations will be better for that individual.

I would think that you could only talk about moderation in things that are not clear rules. Moderation in exercise or things such as that. some things are pretty cut and dry. Casual sex, fornication, not up for debate. Getting drunk, clearly prohibited. The bible is clear on a lot of things. It is the rest that you use moderation and balance in.

The bible says:

But the fruit of the Spirit is love, joy, peace, forbearance, kindness, goodness, faithfulness, gentleness and self- control. Against such things there is no law. (Galatians 5:22-23)

None of these words tend to draw images of somebody flipping out along an extreme. Love may be the most balanced emotion, in that it alone can direct all other emotions without causing internal conflicts. The word self-control also speaks directly to the subject, for it implies that as humans we are tempted towards excessive behavior, that even moralists like someone else pointed out, the Greeks, condemned.

For example, where self-control is not followed, as in the case of excessive drinking, the Bible contrasts this extreme, as opposite to being filled with the Spirit on account of it:

Do not get drunk on wine, which leads to debauchery. Instead, be filled with the Spirit (Ephesians 5:18). 

Interestingly the word debauchery (ἀσωτία) means 'excess', or 'riot'.  Is being excessive not being extreme? This is really what many mean when they say, 'Well that's pretty extreme' often followed by laughter. It means someone hast lost their balance, or 'cool' and is acting to 'excess'. 

Excess often takes the form of excessive emotion that do not seem to match what the circumstances call for. From this standpoint extremism is simply long term emotionalism indicating damaged emotions. Sometimes a perfectionist acting like a theological cop on the beat, is just a sincere person who had a father that did not accept him, so there all stressed out poking themselves into other people business when not invited.

For extremism related to damaged emotions refer to this post as God is in the business of healing us regularly.

What kinds of extremes have I encountered (or have been terribly guilty of)?

• Too much into God's word, neglecting the fruit of the Spirit that makes a person relaxed and loving
• Too much into the Spirit wondering if it 'God's will' to buy this chocolate bar versus that one.
• Too much into church, neglecting family and friends
• Too much  friends and family time, with no time to meet new people at Church
• Too much insistence on traditions of a single church
• Too little respect for anybodies traditions, or the beliefs of previous generations
• Too many opportunities for sharing the gospel where its actually just rude pushiness
• Too little concern and sharing with others about God's love in the gospel
• Too much involvement in politics so that the gospel message takes a back seat while abortion, poverty, etc. grab all the attention.
• Too little concern  for our community or church, resigning everybody to hell as we whistle and skip along our merry way -Too lazy to get a job or work hard in the crummy one that we have
• Too ambitious and greedy in our career to stop to think what matters
• Not believing in the supernatural at all
• Thinking their is a demon lurking around every corner
• Giving all you have to the poor but not calling them to repentance and faith in Christ
• Giving all to yourself and wondering why you don't receive the proper respect for your knowledge and wisdom?
• Judging yourself by standards nobody has ever lived up to
• Congratulating yourself for getting out of bed

Anyway the list could go on forever but love avoids all extremes and balances justice and mercy. It made Jesus tough and tender, serious but relaxed, among many other perfect excellencies of virtue.

The fruit of the Spirit leads to a balanced mind and heart, so 'yes' although the word 'extreme' may not have been common in Biblical days, the Bible inherently avoids all extremes. Extremes are to be avoided unless they are extremely good, like total abandonment to faith, love and hope in Christ. Such a person is extremely balanced all the way around.

The Bible does indeed support the idea of balancing between two extremes, even if only indirectly.

Matthew 7:13-14 (NIV)

¹³ “Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it. ¹⁴ But small is the gate and narrow the road that leads to life, and only a few find it.

Interestingly enough, when I recalled this verse before looking it up, I had this mental feeling that to go to either the left or the right side of the road would be on the path to destruction (likely influenced by this scene in Facing the Giants). However, as I realized when I read it, Jesus is actually talking about two separate roads. So, I had to get the don't-turn-to-the-left-or-right part from somewhere else.

Joshua 1:7 (NIV)

⁷ “Be strong and very courageous. Be careful to obey all the law my servant Moses gave you; do not turn from it to the right or to the left, that you may be successful wherever you go.

Proverbs 4:27 (NIV)

²⁷  Do not turn to the right or the left;
        keep your foot from evil.

In both of these verses (and at least five others), there is this idea that going straight is the only good way. Hence, this can be combined with Matthew 7:13-14 to say that narrow and straight is the road that leads to life, so to turn away from it is to invite destruction. There is also the fact that turning off a road usually wasn't (and isn't, especially at high speed) a great idea, given all the bandits, wild animals, and other dangers were off the road.

So there you have it. Stay on the road and don't turn to the right or left. Balance between two extremes.

(Note: I didn't answer your secondary question. That could easily be its own question.)