Short answer
Worship in Islam means obeying God (﷾) out of love and humility, in every part of life. The Arabic word is 'ibada. It comes from the same root as 'abd, which means a servant. A servant does what his master wills, so worship is more than a ritual. The Qur'an says God (﷾) created people only to worship Him. Prayer, fasting and charity are worship. So is any lawful act done to please God (﷾), even feeding your family. Islam is a complete way of life (deen), and all of it can be worship. Jesus taught the same. He said that calling him Lord is not enough to enter the kingdom. Only the one who does the will of his Father in heaven will enter. That is Islam's answer: worship is a life of obedience to God (﷾).
Full explanation
What the Arabic word means
The English word worship comes from an old word for honour or worth. The Arabic 'ibada shares its root with 'abd, a servant or slave. The classical Arabic dictionaries define it as obedience with humility. Muslims say in every prayer that they worship God (﷾) alone. The Qur'an says this is why people and jinn (unseen beings God (﷾) created from fire) were created.
Gathers the classical Arabic dictionaries. Under the root '-b-d it gives 'abd as a slave or servant, and 'ibada as obedience with submission and humility, service of God, and worship.
“Thee do we worship, and Thine aid we seek.”
“I have only created Jinns and men, that they may serve Me.”
A whole life given to God (﷾)
Islam is a complete way of life (deen), so worship is not only what happens in the mosque. The Prophet (ﷺ) was told to say that his prayer, his sacrifice, his life and his death all belong to God (﷾). He taught that deeds are judged by their intentions. He also taught that a man is rewarded even for the food he puts in his wife's mouth, when he does it to please God (﷾). Work, family life and kindness become worship when they are done for God (﷾) within His law.
“Say: "Truly, my prayer and my service of sacrifice, my life and my death, are (all) for Allah, the Cherisher of the Worlds: No partner hath He: this am I commanded, and I am the first of those who bow to His will.”
Umar ibn al-Khattab heard the Prophet (ﷺ) say that deeds are judged by their intentions, and each person receives what he intended. A man who emigrated for worldly gain or to marry a woman has only what he emigrated for.
Sa'd ibn Abi Waqqas reports that the Prophet (ﷺ) said a person is rewarded for whatever he spends seeking Allah's pleasure, even the morsel of food he puts in his wife's mouth.
Worship keeps God (﷾) in mind
Worship also keeps a person close to God (﷾). God (﷾) tells Moses to establish prayer so that he remembers and praises Him. God (﷾) prescribes fasting so that believers learn self-restraint, and tells them to praise Him often. The Qur'an says Satan works by making people forget God (﷾). It names intoxicants (drink that clouds the mind) and gambling as his tools for turning them from remembrance and prayer. Those who remember God (﷾) after they sin turn back and ask forgiveness.
“Verily, I am Allah: There is no god but I: So serve thou Me (only), and establish regular prayer for celebrating My praise.”
“O ye who believe! Fasting is prescribed to you as it was prescribed to those before you, that ye may (learn) self-restraint,-”
“O ye who believe! Celebrate the praises of Allah, and do this often;”
“The Evil One has got the better of them: so he has made them lose the remembrance of Allah. They are the Party of the Evil One. Truly, it is the Party of the Evil One that will perish!”
“Satan's plan is (but) to excite enmity and hatred between you, with intoxicants and gambling, and hinder you from the remembrance of Allah, and from prayer: will ye not then abstain?”
“And those who, having done something to be ashamed of, or wronged their own souls, earnestly bring Allah to mind, and ask for forgiveness for their sins,- and who can forgive sins except Allah?- and are never obstinate in persisting knowingly in (the wrong) they have done.”
All creation glorifies God (﷾)
A person who worships joins the rest of creation. The Qur'an says the seven heavens, the earth and everything in them declare God's (﷾) glory, though people do not understand how. It tells the Prophet (ﷺ) to celebrate the praises of his Lord and ask His forgiveness.
“The seven heavens and the earth, and all beings therein, declare His glory: there is not a thing but celebrates His praise; And yet ye understand not how they declare His glory! Verily He is Oft-Forbear, Most Forgiving!”
“Celebrate the praises of thy Lord, and pray for His Forgiveness: For He is Oft-Returning (in Grace and Mercy).”
The message of every prophet
The Qur'an says that every messenger was sent with the same message: there is no god but Allah (﷾), so worship and serve Him. Obedient worship of the one God (﷾) is the way of life of every prophet. It is not a new teaching.
“Not a messenger did We send before thee without this inspiration sent by Us to him: that there is no god but I; therefore worship and serve Me.”
Jesus taught the same
Jesus taught this too. He said that calling him Lord will not save a person who does not do his Father's will. Even doing wonders in his name is not enough. The wise man is the one who hears his words and does them. A man asked Jesus what he must do for eternal life, and Jesus told him to keep the commandments. A lawyer recited the command to love God (﷾) and neighbour, and Jesus said: this do, and thou shalt live. He also warned that whoever breaks the least of the commandments, and teaches others to, will be least in the kingdom.
“Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.”
“Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.”
“Therefore whosoever heareth these sayings of mine, and doeth them, I will liken him unto a wise man, which built his house upon a rock:”
“And, behold, one came and said unto him, Good Master, what good thing shall I do, that I may have eternal life? And he said unto him, Why callest thou me good? there is none good but one, that is, God: but if thou wilt enter into life, keep the commandments.”
“And, behold, a certain lawyer stood up, and tempted him, saying, Master, what shall I do to inherit eternal life? He said unto him, What is written in the law? how readest thou? And he answering said, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy strength, and with all thy mind; and thy neighbour as thyself. And he said unto him, Thou hast answered right: this do, and thou shalt live.”
“Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.”
Verses Christians cite
John 6:40 — Gospel of John, chapter 6, verse 40
“And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day.”
Cited for
The will of the Father is that people believe in the Son. So doing the Father's will in Matthew 7:21 means believing in Jesus.
The response
In Matthew 7, Jesus explains his own words. The people he turns away called him Lord and did wonders in his name. Yet he sends them off as "ye that work iniquity". Their deeds condemn them. The one who stands is the one who hears his sayings and does them. John 6:40 also speaks of the will of "him that sent me". Elsewhere in Matthew, Jesus tells a man seeking eternal life to keep the commandments.
Where it lands — In the passage itself, the Father's will is done by obeying, and calling Jesus Lord is not enough.
Galatians 2:16 — Epistle to the Galatians, chapter 2, verse 16
“Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified.”
Cited for
Paul, a first-century Christian missionary, taught that a man is justified by faith in Christ, not by the works of the law. Obedience follows as the fruit of salvation. It is not the way to eternal life.
The response
This is Paul's teaching, not Jesus'. Asked twice what brings eternal life, Jesus answered: keep the commandments, and this do, and thou shalt live. When his disciples asked who then can be saved, Jesus said it was impossible with men but possible with God (﷾). That is God's (﷾) help, not an end to the commandments. He warned that whoever loosens the least commandment will be least in the kingdom. Islam follows the answer Jesus gave. Islam also teaches that no one enters Paradise by deeds alone, without God's (﷾) mercy. In that same saying, the Prophet (ﷺ) told his followers to do good deeds properly and sincerely. So obedience is not a claim to earn salvation, and mercy never sets God's (﷾) law aside.
Where it lands — Asked the way to eternal life, Jesus pointed to the commandments. Paul says no one is justified by the works of the law. A Muslim follows the answer Jesus gave.
Perspectives
Mainstream Christian
Worship is the adoration of the one God, Father, Son and Holy Spirit. Salvation is received by faith in Christ, not earned by keeping the law. Obedience follows as the fruit of that faith.
In John, Jesus speaks of the will of the one who sent him. It is that everyone who sees the Son and believes in him may have everlasting life.
Rebuttal
John 6:40 does join belief in the Son with eternal life, and it is the will of "him that sent me". But in Matthew 7, Jesus turns away people who called him Lord and did wonders in his name. He calls them workers of iniquity. The one who stands hears his sayings and does them. And he told a man seeking eternal life to keep the commandments.
Paul was a first-century Christian missionary. He taught that a man is justified by faith in Christ, not by the works of the law.
Rebuttal
Paul did write this, but Jesus, asked the way to eternal life, pointed to the commandments. Islam agrees that deeds alone save no one, only God's (﷾) mercy. Yet in that same saying the Prophet (ﷺ) told his followers to do good deeds properly and sincerely, so mercy never sets God's (﷾) law aside.
The Islamic perspective
Worship is obedient submission to the will of the one God (﷾), out of love and humility. It covers prayer and fasting. It also covers every lawful act done for Him. It was the message of every prophet, the Prophet Muhammad (ﷺ) last of all.
The Arabic word for worship, 'ibada, shares its root with 'abd, servant, and means obedience with humility.
The Qur'an states that God (﷾) created people and jinn only to worship Him.
Every messenger was sent to call people to worship God (﷾) alone.
The Prophet (ﷺ) said a man is rewarded even for the food he puts in his wife's mouth. This is when he does it for Allah's (﷾) sake.
Any lawful act done to please God becomes worship.
Jesus made doing the will of the Father, not calling him Lord, the condition for entering the kingdom.
Asked the way to eternal life, Jesus twice pointed to God's (﷾) commandments.