Herbert's "Jordan (I)" is very difficult to understand because understanding the poem depends completely upon understanding the allusions that pepper the poem, the allusions that are scattered throughout. Remembering that Herbert was a devout Christian Anglican and minister, after resigning his parliamentary career, it is easier to understand the first and central allusion in the title: Jordan. There are two "Jordan" poems and both discuss writing poetry.
"Jordan" is thought by most critics to allude to the Jordan River that is important to the people of Israel in the Old and New Testaments. In the Old, the people of Israel cross the Jordan to get to the "promised land," and, in the New, Jesus is baptized in the Jordan at the beginning of his ministry. The general opinion, then, is that Herbert is setting up a poem about the divinely inspired potential of poetry as being regenerative and as giving renewal, if, that is, poetry could stop being what he saw it as presently being, which was that poetry was false.
Who says that fictions only and false hair
Become a verse? Is there in truth no beauty?
In the first stanza, Herbert contrasts the Jordan allusion--the potential for poetry to give spiritual renewal--to poetry that is fictitious, false and artificial. There is debate about some of his allusions in this stanza but he seems to be invoking images of sonnets to loved ones who have artificial beauty (false hair) and images of poems that praise this falseness; he seems to be lamenting this falseness in persons and in poetry: "Is there in truth no beauty?" He seems to criticize the structure of poetry, compare it to a winding, circular staircase and suggesting poetic structure is overly complicated. He seems to suggest that in poetry reality is embellished, that it can't be plain reality: "Not to a true, but painted chair?"
It is clear now how allusion is present in every line and through the allusions in the second stanza, Herbert seems to be criticizing poetic conventions and cliches. Many critics take "enchanted groves" as an allusion to the convention of pastoral poetry that praises the rural lives of shepherds and shepherdesses. Herbert seems to see this as part of the falseness of poetic convention and cliched lines, like "purling streams refresh a lover's loves." Ironically, since Herbert is considered a metaphysical poet, the last two lines seem to criticize the conceits of metaphysical poems, which make unusual comparisons between two things to arrive at one truth.
The third stanza alludes, again, to pastorals and to the second stanza itself. He is suggesting that while pastoral poems may go too far from reality, shepherds are themselves "honest people" who should sing as they like. Yet, he says that he rejects poems with riddles to solve and cliched phrases, like "nightingale or spring." The last two lines seem ambiguous to critics. Some say they allude to Herbert who wants to write plain, straightforward poetry. This explanation seems unlikely to other critics who suggest Herbert is further criticizing poets who drop rhyming and write in plain lines without rhyme: "Who plainly say, my God, my King."
This latter opinion makes a good deal of sense as it accords with the syntax of the three lines: envy no man, let them, who. It also accords with what we know of Herbert's poems, which speak honest truth but surely do not do it in a plain and straightforward way.