Focusing on the Byzantine writings in support of the Filioque- addition to the creed, the present study tries to give us the flavour of the Trinitarian reflection in the age of the thirteenth century providing the reader with an analysis of the major arguments advanced by two important figures on the orthodox side, J. Beccos and K. Melitiniotes. In order to refute Photius⁅ position against the Filioque, J. B and K. M left to us a remarkable theological legacy, which must be in any case evaluated objectively. In this effort lies precisely the goal of this paper, namely to assess without confessional prejudices the validity and soundness of their arguments. In addition to that, the paper reveals striking similarities between J. Bs⁅ and K. Ms⁅ thought and the theological reasoning of the Latin West.