This proverb is understood as one speaking about real estate and how land is treated. At least this is language which is used. The "ancient boundary" referred to markers, usually rather large stone markers that were set in place by the previous generation - by which the boundaries of the land were marked. Those who do surveying today do the same thing - except they use deep metal spikes driven into the ground as the markers. All land surveying is done from these markers to make sure that land that is sold and transferred is done legally. So on the surface level this passage is saying that God is against illegal land transactions where one party tries to cheat another out of land by moving the ancient boundary marker to their advantage. But there is more to this proverb than what we see on the surface - there is a principle that is very important for us grasp - especially in light of our ever-changing culture.
There are physical boundaries - and there are social and spiritual boundaries. There are things which our forefathers gave us as a nation - and things which our spiritual forefathers gave us in the church. The question in an ever-changing society is this. When is it right to change things - altering with the times - and when are we dealing with the ancient boundaries that our fathers set for us? Even more important for us in the church there is the question of when is something an enduring spiritual truth - and when is it something a tradition arising out of denominational preference?
First of all, I am not sure that anyone can know all boundaries perfectly. Nations and especially churches are in conflict about such things on a regular basis. What is tradition that is the ancient boundaries - and what tradition is . . . well, just tradition? When it comes to things like this, we need an authority in the matter - and it needs to be outside ourselves. It even needs to be outside of our fathers and forefathers. Therefore we turn to the Scriptures for guidance. The things that are immovable have to do with theology. We can NEVER change our theology. Things like the Deity of Christ - His virgin birth, death, burial, and resurrection can never be altered. Things like the person and nature of God - the Holy Spirit - the veracity of the Bible as God's self-revelation are all non-negotiables. Then we have the negotiable items. These are driven more by basic principles and preferences. One very controversial area in the church is music and ministry styles. There are basic principles of sharing the gospel. The gospel must remain pure - but the style in which we share it will morph as society changes. Wisdom says that there is merit in modernizing our style of ministry - how we share the gospel - ways we interact with people - but the truth of the gospel itself is never to be changed. Similar things exist in musical worship. The principle that God is worshipped and is the center of our thinking and focus cannot be changed. We sing to Him and He is the One on whom we focus in worship. Instruments in worship may change over time - even styles of music will alter over the years, but there is never to be a change in focus.
Traditions can be tricky things. Those who survey land know that it can be a tricky thing to do it properly. At Calvary Chapel we had a problem with survey work that was not done properly for us in a land transaction. The surveyor did not accurately read the set pin for our land. He just guessed at it - and as a result made what seemed at first a minor miscalculation in drawing a line for our land border. The problem became much more pronounced though as the line went out further. Eventually the line which ran for about 100 yards cut off an ever-increasing pie-shaped part of our land. What started out as a small error - got far worse as it widened out. The final result was a large corner of land and a large error - all because he ignored the set pin put down by the fathers. This can happen to us spiritually as well. It may seem like such a small thing - a small change that we are making. It may seem like we're only ignoring a small principle of the Scriptures at first. The problem is that as time progresses, ignoring the ancient set point of our fathers (which they set according to Scripture) will be far worse in the future. Such things won't happen if all we are doing is seting new traditions for a new generation of people. But ignoring Scripture - ignoring the ultimate Father and the boundaries He has set - will cause great harm in the end - and honestly - even in the meantime.