.أوّلُ عِبادةِ اللّهِ مَعرفتُه، وأصلُ مَعرفةِ اللّهِ تَوحيدُه
Awwal [أوّل]: that which is earlier and followed by another—be it material or spiritual, and the other is called “last” [ākhir].
‘Ibādah [عِبادةِ]: utmost humility and obedience to a master.
Ma‘rifah [مَعرفة]: information about a thing; knowledge and acquaintance with its features and effects.
Aṣl [أصل]: something on which another thing is based, and this definition makes sense in the presence of a branch [far‘].
Ma‘rifah or proper recognition of God, the Exalted, is the priority, and then it is followed and joined by servitude, humility and obedience to Him.
Thus, gnosis occupies the first level and there are [different] degrees of gnosis. Worship or servitude is expressed according to one’s degree of gnosis.
And the essence of worship which is humility and obedience will not be realized without the realization of gnosis because so long as man has not recognized the station of greatness, majesty and glory of a Being, humility and obedience [to Him] will not take place, too.
Speaking of gnosis, its foundation is tawḥīd or recognition of the Oneness of God, the Exalted. Unless this root is well entrenched and embedded, one cannot build a foundation on it and derive anything from it.
And the foundation of gnosis of anyone who is not anchored in tawḥīd and openly or secretly focuses his attention to other than the One and Only God is not yet solid, firm and deep-rooted and thus he will not attain [true] gnosis.
So, the main foundation of tawḥīd must be firm and then the branch or twig of gnosis will sprout, and consequently worship or servitude [of God] will take place.
Meanwhile, the word used in the first [part of the] sentence is “awwal” and “aṣl” in the second [part] because aṣl is something which is by itself and the foundations are not separated from one another, just as tawḥīd emanates from gnosis. This is contrary to “awwal” and “ākhir” for awwal is something which comes earlier while ākhir is what follows it, like gnosis which is the awwal and ‘ibādah which follows it.
(An excerpt from ‘Allamah Hasan Mustafawi, Risālah Ma‘rifat Allāh: An Exposition of Imām ‘Alī ibn Mūsā al-Riḍā’s (‘a) Sermon on the Gnosis of Allah, trans. Mansoor Limba (Elzistyle.com), pp. 19-20.