…he feels that, although in principle there is certainly no reason why excerpts from other Sacred Scriptures should not be read in the spiritual part of our Feasts, as this is particularly an occasion when Bahá'ís get together to deepen their own spiritual life, it is, generally speaking, advisable for them to read from their own holy Writings in the spiritual part of the Feast.
– Shoghi Effendi (From a letter written on behalf of Shoghi Effendi, 18 February 1954 to an individual believer; The Compilation of Compilations, vol. I, Nineteen Day Feast)