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It is necessary. 

:See definition for δει [[1163]]
:See definition for οφειλει [[3784]]

δει, the third person of δεω, is commonly used impersonally in classical Greek. This usage is less common, but frequent, in the N.T. δει indicates a necessity in the nature of things rather than a personal obligation; it describes that which must be done. 

οφειλει indicates rather the personal obligation; it is that which is proper, something that ought to be done.