They should, however, repent.
My interpretation of 1 Timothy 4:3 is this (it also is consistent with John Piper’s interpretation, which may not mean anything, I’m just sayin):
God did indeed create food only for believers to enjoy with thanksgiving. That is what the verse says. It was not made for unbelievers, although, because of God’s grace, they may still eat it.
The little known fact though, is that every time they eat His food they are sinning. It was intended for believers who give thanks. An unbeliever cannot truly thank God (without faith it is impossible to please Him).
Oh the depth of our sin! When I was a baby sucking on a bottle I was sinning. Every meal I had as an unbeliever was one more sinful act on my part. We have so little idea how vast our sin problem is.
Due to our shallow understanding of sin, we also have an equally shallow (at best) view of God’s grace. Oh how magnificent it is!