A.T. Robertson Commentary Romans 1:6

"among whom are ye also called [to be] Jesus Christ`s:" — Romans 1:6 (ASV)

Called to be Jesus Christ's (κλητο Ιησου Χριστου). Predicate genitive after κλητο (verbal adjective from καλεω, to call), though it is possible to consider it the ablative case, "called of (or from) Jesus Christ."