Sine Qua Non
A careful reading of Calvin's argument, which appears with astounding frequency against Westphal and others, points to a consistent christological-pneumatological sine qua non in Calvin's theology: the economic identity of Christ and the Spirit. While on the one hand Christ must never be confused onto logically with the Spirit, on the other hand, in their functional or economic identity, Christ must not be separated from his Spirit. (italics original)
Mark Garcia, "Christology", in Lane G. Tipton and Jeffrey C. Waddington, eds., Resurrection and Eschatology: Theology in Service of the Church (Phillipsburg, NJ: PR Publishing, 2008), 426