The classification of the Northwestern dialect of the Mari language, spoken by communities in the Kirov and Nizhny Novgorod regions of the Russian Federation, as well as in the Kilemarsky and Medvedevsky districts of the Republic of Mari El, has long been a subject of academic debate. In this article we present the comparison of the first Gospel written in the Mari language with the Meadow and Hill Mari languages from a morphological perspective. As result we have one isogloss without doublets (PL) that aligns the Gospel with Meadow Mari, and two isoglosses with doublets (IMP.2PL, NEG.PRET), the quantity of which indicates a predominance of Meadow features. Therefore, the analysis of morphological characteristics confirms the data from the graphs and glottochronology, demonstrating that the Northwestern dialect was originally closer to Meadow Mari. However, over time, it acquired features of Hill Mari, thus belonging to the "western" Mari dialects according to G. Bereczki's terminology. This aligns completely with the results obtained from the analysis of the graphs of the first books. Interestingly, the changes in morphology occurred somewhat later. While the [Gospel 1821] already practically coincided with Hill Mari in terms of graphic representation, it was still closer to Meadow Mari in terms of morphology.