They say nothing to anyone. Mark 16:8 reads, "So they went out and fled from the tomb, for terror and amazement had seized them, and they said nothing to anyone, for they were afraid." A footnote in the NRSVue translation says some of the oldest handwritten copies of Mark end the book there. A note in a study Bible, the New Oxford Annotated Bible, says the command to tell the disciples is not carried out in this verse.
In Matthew the women run "to tell his disciples" (Matthew 28:8), and in Luke they tell all this "to the eleven and to all the rest" (Luke 24:9). The scholar Raymond Brown calls the women's reaction in Mark astounding. The same note adds that Mark's audience, who know how the movement around Jesus grew, will know the story continues.
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