What is the ruling on calling the ignorant followers of the people of innovation innovators?

Shaykh Rabīʿ ibn Hādī al-Madkhalī:

Without a doubt, their followers should stop practising the innovation. Those who accompany the people of innovation and defend them are from them. However, you should not say to them, “you are innovators.” Use wisdom and good admonition with them – meaning, if a disbeliever came to him and he said to him, “You are a kāfir” he will not accept his invitation to Islam, likewise the innovator. Do not say “You are a mubtadi’ʿ.” This is from wisdom (not saying such a thing at this time).

For example a Hindu, if you were to say “You are a Hindu.” he would hate you with a severe hatred. The Hindu is more despicable than a Jew or Christian, so do not say to him when you are inviting to Allāh, “You are a kāfir” because he will reject your invitation to Allœh. Rather say “Allāh has sent to us Muammad with guidance and the true religion” and so on. Inform him and bring evidence with truthfulness. Say this and say that, say “I invite you to this religion” but do not say “You are a kāfir.” When you are inviting a Christian to Islam, do not say “You are a kāfir” and when you are calling an innovator (to the sunnah), do not say “You are an innovator (mubtadiʿ)” bring clarity in your invitation.

However, if you are asked, “Are the Jews disbelievers?” then say “Yes” if it is asked of you, “This Jew, is he a disbeliever?” then say “Yes.” Nevertheless, for you to say such a statement while you are calling to Islam is not correct.