Do not move the ancient landmark
that your fathers have set.
This refers to the property lines marking the property belonging to a family. Its application is that we are to honor all contracts and agreements made before us. We do not move property lines; we do not revise contracts - i.e. not without appropriate agreements. An appropriate agreement is not merely a legal one, but one in which we are assuring fairness for the other party.
It means that we take responsibility for the agreements that our fathers and grandfathers and great-grandfathers made. As citizens, we take responsibility for the agreements made by our government both past and present. So do we as members of a church. So do we as members of God's kingdom.
Consider some of the ancient landmarks set by Christ - turn the other cheek, do unto others what you would have them do unto you, go the extra mile. Such agreements we inherited when we took our inheritance. We may not revise the lines; we may not add conditional clauses. As our fathers accepted these lines, so must we, knowing as they did the property awaiting us in heaven.