And the answer to the question is Tertullian:
“But we now advance a step further on, and challenge (as we promised to do) the very Gospel of Marcion, with the intention of thus proving that it has been adulterated. For it is certain that the whole aim at which he has strenuously laboured even in the drawing up of his Antitheses, centres in this, that he may establish a diversity between the Old and the New Testaments, so that his own Christ may be separate from the Creator, as belonging to this rival god, and as alien from the law and the prophets. It is certain, also, that with this view he has erased everything that was contrary to his own opinion and made for the Creator, as if it had been interpolated by His advocates, whilst everything which agreed with his own opinion he has retained.” (Against Marcion, IV.6)
Indeed, it was in a polemical writing against the heterodox teaching of Marcion of Sinope, who taught that the god who was revealed in the Old Testament is different from the god who was revealed in the New Testament. And it was written around 208 AD. To think of it, we should ‘thank’ Marcion for a lot of things. Thanks to him, the church started to think about what books constituted the Christian Scriptures (Marcion was the first person to formulate the canon in 144 AD: Luk, Rom, 1 Cor, 2 Cor, Gal, Eph, Phil, Col, 1 Thess, 2 Thess, Phlm — that’s all; if we’re following Marcion’s canon now, our Bible will be very thin). And, indirectly because of him also, now we called the first part of our Scripture to be the Old Testament and the second part to be the New Testament.