120. It is not for the inhabitants of Medina and the Desert-Arabs around them to stay behind the Messenger of God, nor to prefer themselves to him. That is because they never suffer any thirst, nor fatigue, nor hunger in the cause of God, nor do they take one step that enrages the disbelievers, nor do they gain anything from an enemy, but it is recorded to their credit as a righteous deed. God does not waste the reward of the righteous.
121. Nor do they spend any expenditure, small or large, nor do they cross any valley, but it is recorded to their credit. That God may reward them in accordance with the best of their deeds.
122. It is not advisable for the believers to march out altogether. Of every division that marches out, let a group remain behind, to gain understanding of the religion, and to notify their people when they have returned to them, that they may beware.
123. O you who believe! Fight those of the disbelievers who attack you, and let them find severity in you, and know that God is with the righteous.
124. Whenever a chapter is revealed, some of them say, “Which of you has this increased in faith?” As for those who believe: it increases them in faith, and they rejoice.
125. But as for those in whose hearts is sickness: it adds disgrace to their disgrace, and they die as unbelievers.
126. Do they not see that they are tested once or twice every year? Yet they do not repent, and they do not learn.
127. And whenever a chapter is revealed, they look at one another, “Does anyone see you?” Then they slip away. God has diverted their hearts, because they are a people who do not understand.
128. There has come to you a messenger from among yourselves, concerned over your suffering, anxious over you. Towards the believers, he is compassionate and merciful.
129. If they turn away, say, “God is enough for me; there is no god except He; in Him I have put my trust; He is the Lord of the Sublime Throne.”
Gabriel is ending this chapter by telling Muhammad that the people of Medina and the bedouins,or more nomadic desert arabs have been righteous believers following Muhammad. And in doing so they are provided for by God. These people do the best deeds in the eyes of God in the land of the Arabs. They are not like the people of Mecca who refuse to worship one God and listen to Muhammad.
The believers should not all march out of Medina, some should stay behind to pray at the mosques built and to take care of people in the village. There always needs to be an armed forced on homelands, than out on a battle field.
Islam is considered the religion of peace because it promotes honor in war. You can not fight someone first ever. This line clearly states that you must fight those who fight and try to kill you.
When Muhammad reveals a chapter, since the chapters are revealed one at a time to allow people to digest the information given, some believe it in their hearts and others do not. Because their hearts are not open to the messages, they die in disbelief.
Line 126, Gabriel informs Muhammad that once or twice a person is tested with a major turning event or trial with faith, and if each year they pass they stay on the path to God. Some never repent and never learn from their mistakes Muhammad is told.
Sometimes disbelief causes such disregard for the messages of wisdom that they will think nobody sees them slip off so they don’t have to hear Muhammad speak. But God and the angels see them as they slip off and stray further away from faith.
Muhammad is to say to the people that this is not about him, he is a new messenger that has come to them to help them in their suffering and to show them a new way of life. He reveals to them the path to God because of the concern he has for the people.
But if they still turn away in disbelief, jealousy, rage, or other lower human emotions, then Muhammad is to tell them that God is enough for him and he has put his full trust in God-the Lord of all things.
In one way this Surah/chapter is comfort to Muhammad to be given a glimpse of a way to surpass the mockery and hate that will be thrown his way by the people. He has God and that is all he needs. If people today thought like this, there may be less depressions and suicides regarding what others think about you. All that ultimately matters is what God, the ALL, thinks about you.