Great Sermon! An excellent exegesis of the passage. It raises interesting implications. If "not/but" (ou/alla in the Greek) can be rightly understood as "not only/but also," doesn't that send some sizeable hermeneutical ripples out into the rest of the NT text? Have you considered these?
Covenant of the children John Knox (c. 1514-1572):
"The conviction of the writers of
that Book of Common Order
was thus the biblical perception that
the children of believers
are Christians already,
before
being baptized in their infancy."