Muslims are the ones who usually call themselves the slaves/servant of Allah. Christians seldom call themselves by such except calling themselves sons of God.

But it's amazing why these servitude exist in the Bible.

1. Paul allegedly said in Romans 6:16:
Don’t you know that when you offer yourselves to someone as obedient slaves, you are slaves of the one you obey—whether you are slaves to sin, which leads to death, or to obedience, which leads to righteousness?

This means if we obey God then we are God's slaves!

2. Paul continues later in Romans 6:22:
But now that you have been set free from sin and have become slaves of God, the benefit you reap leads to holiness, and the result is eternal life.

Here we see that those who have been set free from sins become slaves of God!

3. Luke also said in Acts 3:13
The God of Abraham, Isaac and Jacob, the God of our fathers, has glorified his servant Jesus.

Here we see Jesus relegated to being a servant of God by Luke.

(Many Bible translations including NKJV used "Servant Jesus" in the above verse but KJV, Catholic and Aramaic Bible translation used "son Jesus". Amplified Bible decided to used "Servant and son Jesus" showing amplification of course. Young's Literal translation used "His Child Jesus".)

Allah says in the Qur'an that Jesus called himself a slave of Allah.
"He ['Iesa (Jesus)] said: Verily! I am a slave of Allah, He has given me the Scripture and made me a Prophet;"
Qur'an 19:30

So we see Jesus, righteous people are slaves/servants of Allah/God from the Bible and Qur'an!

Are Christians slaves of God too?

Tiiitoo
Zoer
Thorpido

You obey your earthly father, so how come you're not considered a slave ? Please tell us

A slave (servant) is one who has no choice when his master issues an order, many people don't see God as their Master because they feel they can choose to obey or disobey at will.
But JEHOVAH'S WITNESSES believe that we are slaves (servants) of our God JEHOVAH and of Jesus Christ our Lord!
In our heritage (history of God's servants) only faithful and loyal worshipers of God makes themselves slaves (servants) of our God because they've tasted and are fully convinced that our God is good.
So they made themselves His slaves so that whatever His orders we are prepared to OBEY without hesitation!