Allaah says: “Of every troop of them, a party only should go forth, so that they may gain an understanding in the Deen, and that they may warn their people when they return to them, so that they may beware (of evil).” [9:122]
The word, “they” in the part, “so that they may gain” is referring to going forth to gain an understanding of the Deen.
Some scholars went to the other opinion and said that the party which sits behind is the one which gains an understanding in the Deen.
The opinion which was taken as more authentic by Ibn Abbaas, Al-Hasan Al-Basri, At-Tabari, and from the contemporary scholars, Sayyid Qutb, was that those who gain an understanding of the Deen are those who go forth for Jihad.
“The troop which goes forth in the Path of Allaah, that is the one which gains the understanding of the Deen, and that is the one which realizes its hidden beauties, and the Deen reveals its pearls to them.”
Sayyid Qutb said: “Verily, this Deen does not reveal its hidden beauties to the cold sitting Faqeeh who does not struggle to establish this Deen upon this earth. This Deen is not a cake which you can refrigerate in your brain. Rather, this Deen is only understood through struggling to return it unto the sphere of life, and building its society once more.”