Translation:

48. Allah forgiveth not that partners should be set up with him; but He forgiveth anything else to whom He pleaseth; to set up partners with Allah is to devise a sin most heinous indeed.

Notes (Tafseer)

569. Just as in an earthly kingdom the worst crime is that of treason, as it cuts at the very existence of the State, so in the Divine Kingdom, the unforgivable sin is that of contumacious treason against Allah by putting up Allah's creatures in rivalry against Him. This is rebellion against the Creator. It is what Plato would call the "lie in the soul." But even here, if the rebellion is through ignorance, and is followed by sincere repentance and amendment, Allah's Mercy is always open (iv. 17).