The immediate context here is the destruction of Moab, which was located in present day Jordan, ripe for punishment at the hands of the Chaldeans, generally the Babylonians, although used for the Sumerians, Akkadians and Amorites as well.
The Chaldeans were used by God to destroy the Moabites for their numerous infidelities, both physical and spiritual:
Jer 48:11 Moab hath been at ease from his youth…; Jer 48:35 – Moreover I will cause to cease in Moab, saith the LORD, him that offereth in the high places, and him that burneth incense to his gods.
The Chaldeans were not to spare the Moabites like what Saul did to the Amalekites, and Ahab to Benhadad, in these cases the sword was kept back from blood, the enemy not destroyed, and thus the work done deceitfully: dishonestly and insincerely. Non-deceitfulness, on the contrary, is work that sparkles with integrity and diligence.
So what about the preacher man who, from the pulpit, erroneously blazes in the ammunition of a verse like this. Firstly, he is responsible for the misapplication of scripture, for he deviates from the context. He proclaims a curse for deceitfulness only, infact he should also be proclaiming a curse on everybody who refrains from shedding blood by the sword. Secondly, he causes a change in the tenor of the narrative, the new covenant narrative, to create fear and condemnation in those who are hearing him. Not that what he has said is untrue in principle, it is very true, and worthy of emulation if done lawfully (1Ti 1:8 But we know that the law is good, if a man use it lawfully), but he forgets that the letter (the law) with its effects (curses) kills and is incapable of anything productive, for the law makes nothing perfect, but it is the Spirit that gives life. He forgets that in the new covenant every law of the old testament is established by love and not by coercion. For love is the fulfilling of the law, Romans – 13:10.
In the Old covenant:
Deu 27:26 Cursed be he that confirmeth not all the words of this law to do them. And all the people shall say, Amen.
Jer 48:10 Cursed be he that doeth the work of the LORD deceitfully, and cursed be he that keepeth back his sword from blood.
In the New Covenant:
Gal_3:10 & 13 – For as many as are of the works of the law are under the curse: for it is written, Cursed is every one that continueth not in all things which are written in the book of the law to do them. Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree:
Christ has redeemed us from the curse of the law, being made a curse for us. Don’t let anyone tell you otherwise.