Jamieson, Fausset and Brown on Nehemiah 13:4-5

4-5. before this--The practice of these mixed marriages, in open neglect or violation of the law, had become so common, that even the pontifical house, which ought to have set a better example, was polluted by such an impure mixture.

Eliashib the priest ... was allied unto Tobiah--This person was the high priest (Ne 13:28; also Ne 3:1), who, by virtue of his dignified office, had the superintendence and control of the apartments attached to the temple. The laxity of his principles, as well as of his practice, is sufficiently apparent from his contracting a family connection with so notorious an enemy of Israel as Tobiah. But his obsequious attentions had carried him much farther; for to accommodate so important a person as Tobiah on his occasional visits to Jerusalem, Eliashib had provided him a splendid apartment in the temple. The introduction of so gross an impropriety can be accounted for in no other way than by supposing that in the absence of the priests and the cessation of the services, the temple was regarded as a common public building, which might, in the circumstances, be appropriated as a palatial residence.