Allah tells us of His decree for His righteous servants who are the blessed in this world and in the Hereafter, those who will inherit the earth in this world and in the Hereafter. As Allah says:
(Verily, We will indeed make victorious Our Messengers and those who believe in this world's life and on the Day when the witnesses will stand forth.) 40:51
وَمَا أَرْسَلْنَاكَ إِلَّا رَحْمَةً لِّلْعَالَمِينَ (And We have not. sent you but as mercy for all the worlds. - 21:107) عَالَمِينَ is plural of عَالَم which means 'world' and includes all creatures including humans, animals, plants, Jinns etc. In order to properly understand the meaning of the verse one should also understand that the true essence of the entire universe is the invocation of Allah Ta` ala and His worship. When this essence departs from this world and no one is left to worship Allah, everything in it will perish. In other words that is the day when Qiyamah would occur. When it is understood that the invocation and worship of Allah Ta` ala is the essence of all living things, then the reason for describing the Holy Prophet ﷺ as mercy to the Worlds becomes obvious because it is his teachings and the purity of his personal life alone which keep alive the worship of Allah Ta` ala until Qiyamah. That is why he declared "I am the blessing sent by Allah اَنَا رَحمَۃُ مُھدَاۃُ (Ibn ` Asakir quoting Abu Hurairah). Sayyidna Ibn ` Umar , has reported that the Holy Prophet ﷺ said اَنَا رَحمَۃُ مُھدَاۃُ بِرَفعِ قَومِ وَخفضِ آخرِینَ that is "I am the blessing sent by Allah so that I lift up the people (who obey Allah) and humble the other people (who disobey Allah). (Ibn Kathir)
It, therefore, follows that performing jihad against the unbelievers in order to lessen shirk and kufr (unbelief) and in the hope of reclaiming those who transgress the Divine Commands of Allah Ta` ala is an act of mercy.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 106–111
The expanse of the present universe was brought into existence for the purpose of creating a testing ground. Subsequently, when the time comes for creating the world of reaping results (the Hereafter), God will wind up this world, and perhaps with the same material will create another world which will fulfil result-and reward- oriented purposes. The coming into existence of one world (the present world) itself provides sufficient proof of the fact that another world can be brought into existence. In the present world, bad persons attain positions of greatness, but this is only for as long as the period of trial lasts. When the period of trial is over and God’s eternal and perfect world is created, every kind of comfort and honour will be in evidence, but only for those who had proved to be God’s true subjects in this world of trial. This has been set down in detail in the Bible, ‘And do not envy the evil people....Trust in the Lord, and do good; dwell in the land, and feed on His faithfulness. He shall bring forth your righteousness as the light and your justice as the noonday – for evil-doers shall be cut off; but those who wait on the Lord, they shall inherit the earth. The righteous shall inherit the earth and dwell in it forever.’ (Psalms, chapter 37, verses 1, 3, 6, 9 and 27).
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.