The ones cited, on the basis that He inaugurated the New Passover and is the Sacrificial Lamb, and He instituted the New Covenant in His Body and Blood, which we eat and drink, and He took the bread and said: "Eat of this all of you, this is my Body which is broken for you... " Because He did it with the unleavened bread in which He WAS NOT, and BY His Institution, became the Bread which he Instituted AS His Body, and because He Instituted it in a very certain manner recognizable to those not recognizing Him, and because He Commanded His Apostles to go forth AS they had seen Him do, by all this He IS the leaven that is NOT the leaven of the Pharisee, which is hypocrisy, of which the first Pascha which used unleavened bread, is not the Leavened Bread that IS the Body of our Lord of which we partake and eat...
That is the understanding, but you are right, Scripture does not specify leavened bread, but the Church for the first thousand years with no exceptions at all used leavened bread, and I will take their word against those outside their Communion who come along after a thousand years and decide it should be the non-Christian bread of the Jews who killed Christ... We have something much better, missing from the Old Covenant... The Incarnate Christ-logos of God...
And it very well may be, if i go looking it up, that the Church simply leavened the Bread BECAUSE Christ is in it, and wanted to differentiate it from the Pascha of the Jews, and that Christ is then NOT the new leaven... So I may have over-reached in my observation... But He IS the New Pascha, no question about that!
Arsenios