A.T. Robertson Commentary John 20:9

"For as yet they knew not the scripture, that he must rise from the dead." — John 20:9 (ASV)

For (γαρ). Explanatory use of γαρ.

The Scripture (την γραφην). Probably Ps 16:10. Jesus had repeatedly foretold his resurrection, but that was all forgotten in the great sorrow on their hearts. Only the chief priests and Pharisees recalled the words of Jesus (Matthew 27:62ff.).

Must (δε). For this use of δε concerning Christ's death and resurrection see Mr 8:31; Matthew 26:54; Luke 9:22; 17:25; 22:37; 24:7,26,44; John 3:14; 12:34; Acts 1:16. Jesus had put emphasis on both the fact and the necessity of his resurrection which the disciples slowly perceived.