2. Hymns Teach the Faith - for most Catholics, the only catechesis they receive apart from the Sunday homily, are the words of the hymns. They hymns communicate the faith. What we sing we believe. What we believe we sing. Since that is the case it is right that we examine the words of hymns as rigorously as we do the liturgy, and to be frank, many of the modern hymns do not teach Catholic theology at all. In fact, some of the hymns teach Protestant theology, heterodox new age junk, politically correct modernism, and if they do not err in these ways they are simply sentimentalist, wishy washy trash.
The great hymns have stood the test of time. Although many of the hymnists were not Catholic, they were learned pastors and teachers, deeply informed and immersed in classical, historical Christianity, and their hymns communicate the faith fully. Most of the hymns by Wesley and the nineteenth century Anglicans, for example, cannot be faulted for their theology. The theology they express is fully orthodox and acceptable to Catholics, and the lyrics have the advantage of helping Catholics to see their faith from a new perspective–not one that is wrong, but one that they had not seen before.
For example, many of the hymns which originate from non Catholic sources have a special emphasis on the saving work of Christ applied to the individual. This important aspect of the faith is sometimes forgotten by Catholics, or it is experienced and promoted in different ways like the Sacred Heart of Jesus devotion. A sound hymn from a non-Catholic source will bring that learning to light in a new way.
Do we want our children to learn the faith? No better way than to teach them the great hymns of the faith. As they learn the words and music the truths lodge in their hearts. They cannot hum or sing the memorable tune without the splendid words of faith resonating in their heart and mind. A hymn like For All the Saints teaches everything you need to know about the communion of the saints in clear, memorable and moving language. When it is wedded with the stirring tune the child not only learns about the communion of the saints, but takes it to heart. Go here for another hymn and the third reason.