Which monk states that the Old Testament is about the Father and the New Testament about the Holy Ghost Somebody mentioned to me in a conversation that there was a monk in the 13th century who held the belief that the OT is about the Father and the NT is about the Holy Ghost. The death and resurrection are right in the middle of this. The monk was considered a heretic in his time.
However, he forgot the name of the monk.
Is there a monk who held this belief?
The monk mentioned by the O.P. sounds like Joachim of Fiore (c. 1135 – 1202) to me. He was active mostly during the 12th century, but he did make it to the 13th.
Some of his ideas about the nature of the Holy Trinity were refuted posthumously by the Fourth Lateran Council (1215).
However, his doctrine of the “three ages” is a bit different from what the O.P. States. According to Joachim,

The Age of the Father corresponds to the Old Testament.
The Age of the Son corresponds to the New Testament.
The Age of the Holy Spirit corresponds to a future age yet to come.

(See the Wikipedia article for a summary.)