Pursuit of Holiness ch 1-2

What's the point, especially for "once saved, always saved" believers? People that trust in Jesus for Salvation and cleansing of sin are already considered holy by position. So, what's the BIG deal with little sin? What's even the BIG deal with BIG sin? Is there a percieved moral distinction between the two based upon consequence? i.e. the bigger effect the sin has upon the people around me, the worse the sin? OR if that sin has no percieved effect on anyone around me, is it still a sin (i.e. the fleeting heart/bitterness/ inward unholy thoughts?) OR is it because the Lawgiver is BIG and HOLY that we're to respect even the percieved small sins?

We're reading the book the Pursuit of Holiness in our small group through our church, and these seem to be the major questions I encountered in reading the first 2 chapters. I am curious to know your observations as well. These questions aren't namely the biggest points of the book, however they stood out to me the most, for whatever reason... possibly that I justify WAY to many little things.
There's a quote on pg. 20 that says "it is the comprimise on the little issues that lead to the great downfalls". So a dogmatic/skeptic question (if there's such a thing) I would raise is this: Is it the avoidance of the great downfalls that we abstain from sin? OR is it pro-active... i.e. Out of thanksgiving for our salvation and desire to imitate (not take the place) of God that we try with all our hearts to be Holy? "but just as He who called you is holy, so be holy in all you do: for it is written: "Be Holy because I am Holy". 1 Peter 1:15-16.