﴿ فَوَرَبِّ ٱلسَّمَآءِ وَٱلْأَرْضِ إِنَّهُۥ لَحَقٌّۭ مِّثْلَ مَآ أَنَّكُمْ تَنطِقُونَ ﴾

“for, by the Sustainer of heaven and earth, this [life after death] is the very truth - as true as that you are endowed with speech!”

So by the Lord of the heaven and the earth it that which you are promised is as assuredly true as the fact that you have power of speech read mithlu in the nominative as an adjective with the mā being extra; or read mithla in the accusative as being a compound with mā that is to say it is as true as your speech is in reality that is in your knowing it to be true necessarily by its issuing from you.