Just as in the Baptism, where the mystery of the first regeneration was proclaimed, the operation of the whole Trinity was made manifest, because the Son Incarnate was there, the Holy Ghost appeared under the form of a dove, and the Father made Himself known in the voice; so also in the transfiguration, which is the mystery of the second regeneration, the whole Trinity appears--the Father in the voice, the Son in the man, the Holy Ghost in the bright cloud; for just as in baptism He confers innocence, signified by the simplicity of the dove, so in the resurrection will He give His elect the clarity of glory and refreshment from all sorts of evil, which are signified by the bright cloud.
Thomas Aquinas, ST 3.45.4 ad 2.
