I have Protestant friends who say that Jesus died only for those who believe in him (or those who are Christians). What about Muslims, who acknowlege him, or agnostics who don’t truly know him, and other religions?
Jesus died for all mankind. As the apostle Paul says:
For to this end we toil and strive, because we have our hope set on the living God, who is the Savior of all men, especially of those who believe (1 Tim. 4:10).
God desires the salvation of all men (cf. 1 Tim. 2:3-4), thus he died to redeem mankind and offer salvation to all. His death redeemed mankind collectively, so we can say that even non-Christians are redeemed. Because each person has free will, he can choose to reject the salvation offered him as a gift. So, while all are redeemed, it is possible that not all will be saved. Christ’s death is sufficient for the salvation of all, but efficient for the salvation only of those who choose to accept that gift.