Albert Barnes on Matthew 24:12

And because iniquity, etc. The word iniquity here seems to include the cruelty of the Jews and Romans in their persecutions; the betraying of Christians by those who professed to be such; and the pernicious errors of false prophets and others. The effect of all this would be, that the ardour of feeling of many Christians would be lessened. The word wax means to become. It is an old Saxon word, not used now in this sense, except in the Bible. The fear of death, and the deluding influence of false teachers, would lessen the zeal of many timid and weak professors; perhaps also of many real but feeble Christians.