Types of ‘uboodiyyah
Question

                I read some verses which seem to indicate that the slaves of the Most Merciful are only the believers, such as the verse (interpretation of the meaning): 
“And the (faithful) slaves of the Most Gracious (Allaah) are those who walk on the earth in humility and sedateness”
[al-Furqaan 25:63] 
But some other verses indicate that all people are the slaves of Allaah, such as the verse (interpretation of the meaning): 
“There is none in the heavens and the earth but comes unto the Most Gracious (Allaah) as a slave”
[Maryam 19:93] 
How can we reconcile between these verses?.
Praise be to Allah.and blessings and peace be upon the Messenger of Allaah and his family and companions. 
You should note – may Allaah guide you to obey Him – that ‘uboodiyyah (servitude to Allaah) is of two types: a specific type and a general type. 
‘Uboodiyyah in the specific sense means the servitude of love, submission and obedience by which a person is honoured. This is what is referred to in verses such as the following (interpretation of the meaning): 
“Allaah is very Gracious and Kind to His slaves”
[al-Shoora 42:19]
“And the (faithful) slaves of the Most Gracious (Allaah) are those who walk on the earth in humility and sedateness”
[al-Furqaan 25:63]
This kind of ‘uboodiyyah or servitude applies only to the believers who obey Allaah, and is not shared with the kuffaar who rebel against the laws, commands and prohibitions of Allaah. People vary greatly with regard to this servitude; the more a person loves Allaah, follows His commands and submits to His laws, the greater his servitude. The ones who achieved the highest level in this regard were the Prophets and Messengers, and the greatest of all was our Prophet Muhammad (peace and blessings of Allaah be upon him). Hence no one is described as being a true slave of Allaah in the most complete sense in the Qur’aan except him (peace and blessings of Allaah be upon him). Allaah describes him as His slave in the context of Revelation, as He says (interpretation of the meaning): 
“All the praises and thanks be to Allaah, Who has sent down to His slave (Muhammad) the Book (the Qur’aan), and has not placed therein any crookedness”
[al-Kahf 18:1]
And in the context of the Isra’ [Night Journey] (interpretation of the meaning): 
“Glorified (and Exalted) be He (Allaah) [above all that (evil) they associate with Him]
Who took His slave (Muhammad) for a journey by night from Al-Masjid Al-Haraam (at Makkah) to Al-Masjid Al-Aqsa (in Jerusalem)”
[al-Isra’ 17:1]
and in the context of supplication (interpretation of the meaning): 
“And when the slave of Allaah (Muhammad) stood up invoking Him (his Lord Allaah) in prayer they (the jinn) just made round him a dense crowd as if sticking one over the other (in order to listen to the Prophet’s recitation)”
[al-Jinn 72:19]
and there are other similar verses. 
All honour is to be found in achieving this servitude in the fullest sense, which can only be achieved by acknowledging one’s complete dependence on Allaah and feeling completely independent of other people. This can only be achieved when a person combines love for Allaah with fear of Him and hope for His bounty and reward. 
With regard to servitude in the general sense, no created being is excluded from this. It is called compulsory servitude. In this sense all people are slaves of Allaah and are subject to His rulings, and His decree is carried out on them. No one has the power to do harm or do good for himself except by the Leave of his Lord and Sovereign Who controls him. This is the kind of servitude that is referred to in verses such as the following (interpretation of the meaning): 
“There is none in the heavens and the earth but comes unto the Most Gracious (Allaah) as a slave”
[Maryam 19:93]
This kind of servitude does not imply any kind of honour. Whoever turns away from the specific type of servitude mentioned above is still subjugated to servitude in the general sense, and cannot be excluded from that in any circumstances. All creatures are the slaves of Allaah by His will, so whoever does not want to be a slave of Allaah by choice is still a slave of Allaah by compulsion. 
We ask Allaah to make us among His sincere slaves and close friends, for He is All-Hearing, Ever-Close and Ever Responsive. And Allaah knows best. May Allaah send blessings and peace upon His slave Muhammad and upon all his family and companions. 
See Mafaaheem yanbaghi an tusahhah by Shaykh Muhammad Qutb, 20-23, 174-182; al-‘Uboodiyyah by Shaykh al-Islam Ibn Taymiyah.