Symbolism of becoming a chalice (metaphor): Connects to the previous image of Handmaid's needing to empty themselves of all to become a perfect vessel. Being nothing but a container, the chalice symbolises this as it is formally used religiously to hold sacramental/holy wine -- just as Offred is allowing herself to be emptied, mind and body, to carry something sacred (a child) and none more.

Also, because of the communion, it also means that she is turning herself over to God (drinking the blood).

Also, the shape of the chalice is like one of a woman's uterus.

I would be curious to see how the stroke is connected to the mental alteration in behavior that was depicted by the priest as discussed here. Might there be the case that there is more to the story, I am not sure if that had something to do with the boy as well, as he seems to be involved with the chalice

See https://en.wikipedia.org/wiki/Tassilo_Chalice

See also: https://en.wikipedia.org/wiki/Ardagh_Hoard

See also: https://en.wikipedia.org/wiki/Ardagh_Hoard

See also: https://en.wikipedia.org/wiki/Ardagh_Hoard