/summa/TP/TPII/TP_Q17:
We must now consider what pertains to Christ's unity in common. For, in their proper place, we must consider what pertains to unity and plurality in detail: thus we concluded (Q[9]) that there is not only one knowledge in Christ, and it will be concluded hereafter (Q[35], A[2]) that there is not only one nativity in Christ.
Hence we must consider Christ's unity (1) of being; (2) of will; (3) of operation.
Under the first head there are two points of inquiry:
(1) Whether Christ is one or two?
(2) Whether there is only one being in Christ?