'If any man thinks himself to be a prophet, or spiritual, let him take knowledge of the things which I write to you, that they are the commandment of the Lord. But if any man is ignorant, let him be ignorant (or 'he is not recognised').'
Manuscripts of equal weight contain either the present passive indicative or the present active imperative, thus 'he is not recognised' or 'let him be ignorant'.
So Paul now challenges those who claim authority, if they really are prophets or spiritual, to consider what he says and recognise that it is the commandment of the Lord. He is claiming that his letter is a 'revelation' directly from Jesus Christ. If they do not so recognise it they are merely showing their ignorance in spiritual things. So they may choose. Agree with what God has shown him, or manifest that they have no true spirituality but are spiritually ignorant. In which case they may go on in their ignorance for they have no place in the true church of Christ (or they simply are not recognised).