19:30a

But: There is some contrast here between two types of people. The people who leave their possessions and family for Jesus’ sake are contrasted with the people who are first in this world. The Berean Standard Bible indicates this contrast with the word But.

many who are first will be last: In this context, the Greek word that the Berean Standard Bible translates as first means “most important.” The Greek word that the Berean Standard Bible translates as last means “least important.”

Here are some other ways to translate this clause:

many who are high/great now will become low/humble
-or-
many people who are big now will be small in the future
-or-
many people who are respected much now will be respected little in the future
-or-
Many who have the highest place now will have the lowest place in the future (New Century Version)

19:30b

and the last will be first: Some words are implied but not repeated here. The complete clause would be:

and many people who are last will be first

Here are some other ways to translate these words:

many who are low/humble now will become high/great
-or-
many people who are small now will be big in the future
-or-
many people who are respected little now will be respected much in the future

-or-

many who have the lowest place now will have the highest place in the future (New Century Version)