Even a scholar can not declare someone Kafir, not unless that scholar is a higher authority than Allah (astaghfirullah)
Yes its only Allah (swt) who knows best , but scholars can pass judgement based on Quran & sunnah , in cases of apostasy & blasphemy ............in an Islamic state off course.
No one may be judged to be a kaafir or faasiq except the one whom the Qur’aan and Sunnah indicate is a kaafir or faasiq.
Guidelines on takfeer (ruling someone to be a kaafir) - islamqa.info
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