Two women show up for church, dressed essentially identically: mid-calf length dresses with a "modest" neckline, hose, and low-heeled pumps. Hair is neatly done, make-up well-applied but not obvious. If you were to see these women on a daily basis, you'd probably see variations on the theme, pants some days, sneakers maybe, but nothing that anyone would find particularly immodest.
But their hearts are in two different places -- one, with a heart to worshipping God, and not setting a stumbling block for her brothers and sisters in Christ; the other with a heart to show off just how "modest" she is. The first has a thriving relationship with Christ, and her wardrobe, lifestyle choices, and daily life reflects that. The second has a list of rules that she tries very hard to live by, maybe she's a Christian, maybe she just tries to look like one.
Here's the thing -- everything we do is to be done with our heart. Modesty isn't about how we dress. Modesty is about not drawing undue attention to ourselves. Does this impact our clothing choices? It should. Does it impact whether or not we audition for the solo at church? It should.
If our heart is not in truly glorifying God, then anything we do is truly just self-glory. It is idol worship, and the idol is ME.
At the heart of it all is the heart. Where is my heart? To Whom (or whom) does it belong?