Why are most people on Earth not Muslim, given our knowledge that God's love for His creation exceeds a mother's love for her child seventyfold, and is it true that only Muslims, specifically Sunnis, will enter Paradise?

There is no basis for the idea that God's love for His creation is seventy times greater than a mother's love for her child. Allah loves those who do good (muhsinin), the pious (muttaqin), those who repent (tawwabin), those who purify themselves (mutatahherin), the patient (sabirin), those who rely on Him (mutawakkilin), and the just (muqsitin). He has condemned the oppressors (dhalimin), the defiantly disobedient (fasiqin), and the disbelievers (kafirin). As for His mercy, it is established in the Hadith that He is more merciful to His servants than a mother is to her child.

The reason most people on Earth are not Muslim is their choice of disbelief over Islam, despite the clarity of truth, evidence, and proofs of Islam's authenticity. Allah the Almighty did not approve of disbelief for His servants; rather, He showed them the path of truth and falsehood, gave them the choice between faith and disbelief, and promised believers Paradise while threatening disbelievers with Hellfire.

Everyone who believed in their prophet and affirmed their messenger will enter Paradise. After the prophethood of Prophet Muhammad (peace be upon him), only a Muslim enters Paradise. As for the Ahlus-Sunnah, they are the saved sect that follows the truth and does not declare as disbelievers those Muslims who differ with them but died upon monotheism. As for the people of innovation (Ahlul-Bid'ah) and the disobedient monotheists, their affair is with Allah; if He wills, He will punish them, and if He wills, He will forgive them, and their ultimate destination is certainly Paradise.

It is from Allah's mercy that He does not punish with Hellfire those to whom the call of Islam did not reach. These will be tested on the Day of Judgment.