This verse was revealed towards the end of Prophet Muhammed’s (p) life-time in Madinah (Mawdudi). 
“The (true) believers are those only who believe in Allah and His messenger and afterward doubt not, but strive with their wealth and their lives for the cause of Allah. Such are the sincere.” – Quran 49:15
49:15 – The true believer in the sight of God Almighty is the one who believes in HIM and his Prophet (p). After they believed, they have no doubt in Islam and strive with their wealth and lives in the HIS cause. This passage has two meanings: One being, the believers have to strive to do good by obeying and following God’s commandments. Secondly, it refers to believers fighting physically against oppression and tyranny. This is why the word ‘Jihad’ used here has two fold meanings (Tafsir al-Jalalayn and Tanwir al-Miqbas min Tafsir Ibn ‘Abbas).
For further details on the Arabic word ‘Jihad’, please click on the following article which will give you various meanings of it, not what the media have you believe it means: “Arabic Word ‘Jihad’ And Its Meaning“.
Tafsir Ibn Kathir:
“(who have believed in Allah and His Messenger, and afterward doubt not) They do not have doubts and their Faith was not shaken. Rather, their Faith remained on conviction, (but strive for the cause of Allah with their wealth and their lives) meaning, they gladly gave away their life and the most precious of their wealth in obedience to Allah as a means of seeking His pleasure…” (Tafsir Ibn Kathir – online source)
Tanwir al-Miqbas min Tafsir Ibn Abbas:
“Allah then explained the traits of those who are true in their faith, saying: (The (true) believers) who are sincere in their faith (are those only who believe in Allah and His messenger and afterward doubt not) in their faith, (but strive with their wealth and their lives for the cause of Allah) in obedience to Allah. (Such are the sincere) who are true in their faith and striving.” (Tanwir al-Miqbas min Tafsir Ibn Abbas on Surah 49:15 – online source)
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 Sayyid Abul Ala Maududi:
“Traditions show and the subject matter of the Surah also supports the same that this Surah is a collection of the commandments and instructions sent down on different occasions, which have been put together because of the relevancy of the theme. Moreover, the traditions also show that most of these commandments were sent down during the final stage of the Holy Prophet’s life at Madinah. For instance, about verse 4 the commentators state that it was sent down concerning the Bani Tamim whose deputation had arrived in Madinah and started calling out to the Holy Prophet from outside the apartments (hujurat) of his wives, and according to all biographical books on the Holy Prophet’s life this deputation had visited Madinah in A. H. 9. Likewise, about verse 6 a large number of the traditions of Hadith confirm that it was sent down concerning Walid bin Uqbah whom the Holy Prophet had sent to collect the zakat from the Bani al-Mustaliq, and it is well known that Walid bin Uqabah had become a Muslim on the conquest of Makkah.” (Sayyid Abul Ala Maududi – Tafhim al-Qur’an – The Meaning of the Qur’an – Online Source)