“Al hooi" (all hay; in modern English all hooey and in Spanish nonada) is a translation of Martin Luther’s stroern (straw). In the introduction to his 1522 translation of the Bible, Luther called the Epistle of James, which supports the doctrine of salvation by works, “a real epistle of straw” (eyn rechte stroern Epistel). Luther’s downgrading of this and other parts of the New Testament was regarded as heretical, although it should be noted (even though the issues are complex and I am not expert in these matters) that as of 1999 the Catholic church no longer regards the doctrine of salvation by grace as heretical. In any event the haywain pictures are connected not just to Luther’s discourses in general (“all hooey”), but also specifically to his calling part of the Christian Bible “straw.”
The following is from Luther’s introduction:
Summa, Sanct Johannis Euangeli vnd seyne erste Epistel, Sanct Paulus Epistel, sonderlich die zu den Romern, Galatern, Ephesern, vnnd Sanct Peters erste Epistel, das sind die bucher, die dyr Christum zeygen, vnd alles leren, das dyr zu wissen nott vnd selig ist, ob du schon kein ander buch noch lere nummer sehest noch horist, Darumb ist sanct Jacobs Epistel eyn rechte stroern Epistel gegen sie, denn sie doch keyn Euangelisch art an yhr hat, Doch dauon weytter ynn andern vorrheden.
The reason for making the paintings look like the work of Hieronymus Bosch, and for the fake Hieronymus Bosch signatures, was to make it appear as though they came from a more innocent age, from before Martin Luther had begun to publish what the artist of the haywain pictures evidently considered to be his heresies. The paintings were made to be obscure in their meaning so as not to introduce new audiences to Luther’s ideas. The parts of the paintings that make sense to any viewer show people engaged in good works, for instance the child leading a blind man. The hay represents a good thing (part of the Bible) that Luther said was of no value, and this is why the painting shows nuns faithfully collecting hay and bringing it to their superior. The deception implied by the fake Hieronymus Bosch signature was minor compared to the potential harm of publicizing a heresy that threatened the Catholic church.