Several narrations warn against pursuing Islamic knowledge for insincere purposes. The Prophet (peace be upon him) said: "Do not acquire knowledge to compete with scholars, to dispute with the ignorant, or to claim the best seats in gatherings — for whoever does so, the Fire awaits." [Ibn Maajah, 254]
These narrations caution against learning with corrupt intentions — where the primary goal is showing off or engaging in fruitless arguments rather than seeking truth.
The proper motivation for studying Islam includes acting upon the knowledge oneself, conveying it to others, and defending the faith against those who attack it. Allah says: "Do not yield to the disbelievers, and strive against them with the Quran, a great striving." [al-Furqaan 25:52]
Therefore, anyone who studies Islam with the genuine intention of countering false claims, guiding the confused, and defending the truth possesses a praiseworthy motivation.
When debate becomes necessary in the course of inviting people to Allah, it is permissible — provided one follows the Quranic directive: "Invite to the way of your Lord with wisdom and good counsel, and reason with them in the most courteous manner." [an-Nahl 16:125]
In summary: Studying Islam to defend the faith and call people to the truth is a commendable intention. However, students should ensure they do not neglect their individual religious obligations — such as understanding their beliefs, prayers, and fasting — while specializing in fields that address communal needs.