Isn't claiming to do "God's" work, a SIN?

Probably, since it is always claimed in a prideful manner.

that would very much depend upon what was being claimed to be "the work of god"......Those of The True Christian Faith are commissioned by God to do certain "work" in His Name.

Sin is any act against The Word and Will of God for man-kind

No, not necessarily.

If someone claimed to be doing God's work, and the truth was they actually WERE doing God's work, by what standard do you use to claim that they were NOT doing God's work?

I do my best (by God's grace), unless it something that is abundantly clear not God's work because it disagrees with God's inerrant word of the Bible, to not judge my fellow children of God. I am not the Master. I fail at this, but God is sanctifying me.

It is not taking God's name in vain if they truly ARE doing God's work.

Unless you mean it like that no.... Actually us Christians are happy to work for God so we tell people about how exciting it is to be a Christian. And plus have you ever heard of this... For all have sinned and come short of the glory of God.

Sin Is Transgressing ABBA YAHUEH ALMIGHTY'S 10 Commands...

Plane And Simple...

no, to do God's work is to believe in Jesus death burial and resurection by faith.

More like a delusion.