this world (literally this age): Many Jewish people understood history as divided into two ages. One age is the present age, marked by sin and death. The other age is the age of salvation, when God puts things right.
Early Christians adopted this way of thinking. They understood the coming of Christ as the beginning of the new age of salvation.
However, the new age did not replace the old age right away. The old way of thinking and living continues. It still tempts believers, even those who want their lives to match the values of the new age.