Albert Barnes Commentary Jeremiah 12:5-6

"If thou hast run with the footmen, and they have wearied thee, then how canst thou contend with horses? and though in a land of peace thou art secure, yet how wilt thou do in the pride of the Jordan? For even thy brethren, and the house of thy father, even they have dealt treacherously with thee; even they have cried aloud after thee: believe them not, though they speak fair words unto thee." — Jeremiah 12:5-6 (ASV)

Yahweh rebukes Jeremiah’s impatience, showing him by two proverbial sayings that there were still greater trials of faith in store for him. Prosperous wickedness is after all a mere ordinary trial, a mere “running with the footmen;” he will have to exert far greater powers of endurance.

And if in the land ... - Rather, “and in a land of peace thou art secure; but how wilt thou do amid the pride of Jordan?” If you can feel safe only where things are tranquil, what will you do in the hour of danger?

The “pride of Jordan” is taken to mean the luxuriant thickets along its banks, famous as the haunt of lions (Jeremiah 50:44; Zechariah 11:3). What will the prophet do when he has to tread the tangled maze of a jungle with the lions roaring round him?

Jeremiah 12:6. Called a multitude - Rather, “called aloud.” . In all this Jeremiah was the type of Christ (Mark 3:21; John 7:5).