Of course, those with only the OT scriptures might indeed need to have someone explain that Jesus was the Messiah being talked about. This is exactly what the Apostles began doing at Pentecost.
But now we have the New Testament which clarifies that Jesus is the Messiah that was cut off for our salvation. People can respond in faith to the written word just as they could to the spoken word at Pentecost. The scriptures are sufficient today to reveal Christ and thoroughly equip the believer. Ii Timothy 3:15-16
A man stranded on a deserted island with only a Bible can come to saving faith in Christ without the church or an organization of men.