See Definition for dei 1163
See Definition for ofeilei 3784
dei, the third person of dew, is commonly used impersonally in
classical Greek. This usage is less common, but frequent, in the N.T.
dei indicates a necessity in the nature of things rather than a
personal obligation; it describes that which must be done.
ofeilei indicates rather the personal obligation; it is that which
is proper, something that ought to be done.
