Verse 31 does not begin with the word for.
But the use of but, however
indicates that verse 31 contains a type of warning for the disciples. They now think themselves to be far and away the first as early followers, but they must realize that the last (such as the rich young man) could come ahead of them. Compare Judas, one of the twelve, with Paul, a persecutor of the church.
What seems impossible with man remains impossible also with disciples: it is possible only with God that the rich and the followers are saved. Leaving possessions behind and following may not become a new wealth wherein one trusts; how difficult, then, is it for the first to enter the kingdom!1
31 Plusieurs des premiers seront les derniers, et plusieurs des derniers seront les premiers.