The Church has always taught, and continues to teach, that the documents of Pope Pius X on Church music and sacred music are still valid. The statements of Vatican II also affirm that Church music must be sacred, not worldly. This means it should express the sacredness through both the melodies and the instruments used. These are still valid norms of the Church, but unfortunately, they are not always observed.
Follow-up Question: Should the guitar, being a sacred or secular instrument, be used at Mass?
Common sense and the general apprehension of most people tell us that the guitar is so closely associated with entertainment on stage, in restaurants, or in recreational settings that it creates an atmosphere of informality. Therefore, it is not suitable for creating a sacred environment. The Church has stated that the principal instrument should be the organ (or piano). So, if possible, Vatican II intended for the organ (or piano) to be the main instrument. If there is no organ, then we should sing without instruments.