The beauty of holiness refers to the glorious and mortal perfection of God.
Worshipping the lord in the beauty of his holiness means to approach God with a pure heart, showing his character through love, kindness, humility and removing yourself from sin and keeping yourself holy.
One has to be divinely purify, because is the radiance of God own nature which set apart blameless and majestic.
It's a command to worship God not just outwardly but with an inner transformation recognizing his sacredness.
For one to worship in God's holiness you have to change your thinking to align with God's goodness, mercy and love.
Live righteously, control your anger, practice self control and seek peace with others.
Cloth your soul with humility, kindness and love rather than mere outward appearance.
Set yourself apart by making decisions in line with God's will, separating yourself from the world sinfulness.