59 Besides the organ, other instruments can be called upon to give great help in attaining the lofty purpose of sacred music, so long as they play nothing profane nothing clamorous or strident and nothing at variance with the sacred services or the dignity of the place. Among these the violin and other musical instruments that use the bow are outstanding because, when they are played by themselves or with other stringed instruments or with the organ, they express the joyous and sad sentiments of the soul with an indescribable power. Moreover, in the encyclical Mediator Dei , We Ourselves gave detailed and clear regulations concerning the musical modes that are to be admitted into the worship of the Catholic religion.
Source: Musicae Sacrae (Vatican.va)