Isaiah holding a phylactery bearing the inscription: "This is what the Virgin will conceive." Allusion to the prophecy in chapter 7  (Is 7, 10-16) of the book of Isaiah.
<a href="//commons.wikimedia.org/wiki/User:Daniel_VILLAFRUELA" title="User:Daniel VILLAFRUELA"> Daniel VILLAFRUELA</a>.
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>