Peter probably means here that Christ died in order to create a close relationship between us and God. (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here "flesh" refers to Christ's body; Christ was physically put to death. This can be stated in active form. Alternate translation: "People put Christ to death physically" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-activepassive]])
Possible meanings are 1) by the Holy Spirit's power or 2) in a spiritual existence. This means Jesus died a human being, but he was raised as a spiritual being. Jesus had a physical body when he came back to life, but his new body would never die again. Alternate translation: "in the spirit" or "in a spiritual existence"