Dach, the first commandment is Thou shalt have no other God but me, which I would think is the same in Islam. Jesus also said this is the first and the greatest commandment, also that we should treat others as we would treat ourselves. Of course, we can argue how should someone with low self-worth treat others but that’s just semantics. The only unforgivable sin in Christianity is “sins against the Holy Spirit” – to whit, denying God once you know Him (not too dissimilar to Islam, as you have just stated). You cannot deny someone you don’t know.