If you don’t help him, verily Lord Allah did help him when those who disbelieved expelled him, the one among the two, while both were in the cave, when he said to his companion: “You don’t worry, indeed, Allah is with us". So, Allah sent down His Tranquility upon him and strengthened him with forces which you can’t see, thus He made the word of those who disbelieved lowest while the words of Allah remained the Highest, Allah is the Exalted, the Wise.
This verse describes the scenario when the disbelievers at Mecca decided to kill Messenger Muhammad as explained in verse 8: 30. Messenger Muhammad and his companion Aboobacker escaped from them and hid in Thawr cave.
Since Adhikr—the Safeguard—is formed as Book today, whoever holdfasts It, he did holdfast Lord Allah, and he is guided into the Straightpath as explained in verses 3: 101 and 5: 48. Even though Messengers like Moses, Jesus and Muhammad had to migrate from their homeland due to protests and threat from the enemy, they taught: “The believers should be proud representatives of Lord as they have been taught by the Exalted Book and as they have been teaching it” as explained in verses 3: 79-80. The believer by utilizing Adhikr—the Food, Cloth, and Sight of the human soul— will transform his Jinn soulmate into a believer, and thus he keeps Allah in his heart as explained in verse 8: 48. So he will have Tranquility from Lord and need not be fear anyone or anything. See explanations 2: 136; 8: 7-8 and 9: 26.