In Colossians 3:15, Paul admonishes us to let the peace of Christ rule in our hearts. A prominent view in Christian circles is that, when we are faced with a decision, we will know if we are following God’s will by whether or not we feel an inner peace about the situation.
We will talk more about various views on knowing God’s will in the future, but the question at hand is whether Paul is talking about our decision-making process here.
Here’s a useful exercise that we have used before. Try substituting another word or phrase that is synonymous with the word in question and see if it fits the sense of the passage. (Hint: often there are several meanings of the word, so try this for each of them.) I’m sure there are other possibilities, but I have proposed two. Feel free to try this exercise with as many contenders as you can think of.
Possibility #1: “Let the [calm inner spirit that confirms your decisions] rule in your hearts, since as members of one body you were called to [calm inner spirit that confirms your decisions]."
Possibility #2: “Let the [unity/harmony] rule in your hearts, since as members of one body you were called to [unity/harmony]."
Another test we often use is to look at context. What is Paul saying in the passage that this verse comes from? Is it about decision making, interpersonal relations, wars, Woodstock, or is some other notion of peace in view? (You can read the whole chapter here.)
Prior to Col 3:15 we see that - in Christ - there is no distinction between Jew and Gentile, circumcised and uncircumsized, or slave and free. We see Paul saying don’t lie... be nice... get along. Then we hit 3:15. After that he follows up by talking about us all being "one body". Given this context, his instruction about being at peace makes the most sense when thinking in terms of community. There is nothing about decision-making. Nothing about “having a peace about it”. Just a series of instructions about how we ought to relate to one another in the context of the church.
As we have seen before, if your use doesn't fit the context and you can't make sense of the passage by using alternate phrases, you probably are forcing a meaning that wasn’t intended