Like many old hymns, there are many versions of this one tweaking a word here or there. My favorite goes:

Lord, keep us steadfast in Your Word;
Curb those whose feign by craft or sword
Would wrest the kingdom from Your Son
And bring to naught all He has done.

A "feign" is a deceptive scheme. Those whose feign by craft or sword. Yes, around us today are people who have deceptive schemes to wrest the kingdom from Christ. Some scheme to do so by sword and those are, perhaps, more obvious. Those whose feign is by craft, however, are much more common to us this day in America and, dangerously, less obvious.

But bear in mind that the original feign to separate man from God was by craft, "Did God really say that" Today's attack, though, is usually, "Did Jesus really do that?" Did Jesus really save you completely... or is there now something you have to do to complete the accomplishment? When you answer the later, you bring to naught all that he has done.