Satan: See 4:2; 10:18; 11:15. Satan is the accuser of God’s people (see Job 1:7; 2:2). He would test Simon Peter and the rest of the apostles.
has asked to sift: People would sift wheat to separate grain from the parts you do not eat using a device called a sieve. This image represents extreme testing (see Isaiah 30:28; Amos 9:9).
all of you: The Greek here is plural. It refers to all the disciples. Jesus predicted Peter's denial and the difficult experiences of the other disciples.