Sentence:  The motive for <WSD>troping</WSD> the introit was twofold. Firstly there was the desire to add colour, mystical fervour, to the restrained, matter-of-fact Roman rite. Besides this psychological reason there was a practical one. The introit was sung by the choir while the celebrants proceeded towards the altar to officiate at mass. This part of the ritual lent itself very readily to embellishment and expansion. 
Meaning of the word marked with <WSD> tags:  To use, or embellish something with, a trope.