The adoption of the sons of God is through a certain conformity of image to the natural Son of God. Now this takes place in two ways: first, by the grace of the wayfarer, which is imperfect conformity; secondly, by glory, which is perfect conformity, according to 1 John 3:2: "We are now the sons of God, and it hath not yet appeared what we shall be: we know that, when He shall appear, we shall be like to Him, because we shall see Him as He is." Since, therefore, it is in baptism that we acquire grace, while the clarity of the glory to come was foreshadowed in the transfiguration, therefore both in His baptism and in His transfiguration the natural sonship of Christ was fittingly made known by the testimony of the Father: because He alone with the Son and Holy Ghost is perfectly conscious of that perfect generation....
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, 3.45.4c&ad2.
