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Q: Can I use the knife in my office’s kitchen to cut a bagel? I have no idea if the knife was previously used by one of the non-Jews to cut something non-kosher.
A: If the bagel is cold and the knife is clean, the bagel cut with that knife will remain kosher even if the knife had previously been used to cut hot non-kosher foods. Accordingly, one might think that they could cut their bagel with the knife. However, due to the possible mistakes that may occur, Rema (OC 451:1 and YD 121:5 & 7) cites a custom to not use non-kosher utensils in this manner on an ongoing basis. This means that if occasionally (i.e. less than once a month) you need to cut a bagel or other cold food with the clean non-kosher knife youd be allowed to, but you shouldnt make it a regular practice to do so.