The Holman Bible Dictionary, in its article "Church," explains the background of the word church (emphasis added throughout):
"Church is the English translation of the Greek word ekklesia. The use of the Greek term prior to the emergence of the Christian church is important as two streams of meaning flow from the history of its usage into the New Testament understanding of church.
"First, the Greek term which basically means 'called out' was commonly used to indicate an assembly of citizens of a Greek city and is so used in Acts 19:32-39 Acts 19:32-39  Some therefore cried one thing, and some another: for the assembly was confused: and the more part knew not why they were come together.
 And they drew Alexander out of the multitude, the Jews putting him forward. And Alexander beckoned with the hand, and would have made his defense to the people.
 But when they knew that he was a Jew, all with one voice about the space of two hours cried out, Great is Diana of the Ephesians.
 And when the town cleark had appeased the people, he said, You men of Ephesus, what man is there that knows not how that the city of the Ephesians is a worshipper of the great goddess Diana, and of the image which fell down from Jupiter?
 Seeing then that these things cannot be spoken against, you ought to be quiet, and to do nothing rashly.
 For you have brought here these men, which are neither robbers of churches, nor yet blasphemers of your goddess.
 Why if Demetrius, and the craftsmen which are with him, have a matter against any man, the law is open, and there are deputies: let them accuse one another.
 But if you inquire any thing concerning other matters, it shall be determined in a lawful assembly.
American King James Version×. The citizens who were quite conscious of their privileged status over against slaves and noncitizens were called to the assembly by a herald and dealt . . . with matters of common concern. When the early Christians understood themselves as constituting a church, no doubt exists that they perceived themselves as called out by God in Jesus Christ for a special purpose and that their status was a privileged one in Jesus Christ (Ephesians 2:19 Ephesians 2:19Now therefore you are no more strangers and foreigners, but fellow citizens with the saints, and of the household of God;
American King James Version×).
"Second, the Greek term was used more than one hundred times in the Greek translation of the Old Testament in common use in the time of Jesus. The Hebrew term (qahal) meant simply 'assembly' and could be used in a variety of ways, referring for example to an assembling of prophets (1 Samuel 19:20 1 Samuel 19:20And Saul sent messengers to take David: and when they saw the company of the prophets prophesying, and Samuel standing as appointed over them, the Spirit of God was on the messengers of Saul, and they also prophesied.
American King James Version×), soldiers (Numbers 22:4 Numbers 22:4And Moab said to the elders of Midian, Now shall this company lick up all that are round about us, as the ox licks up the grass of the field. And Balak the son of Zippor was king of the Moabites at that time.
American King James Version×), or the people of God (Deuteronomy 9:10 Deuteronomy 9:10And the LORD delivered to me two tables of stone written with the finger of God; and on them was written according to all the words, which the LORD spoke with you in the mount out of the middle of the fire in the day of the assembly.
American King James Version×). The use of the term in the Old Testament in referring to the people of God is important for understanding the term 'church' in the New Testament.
"The first Christians were [mostly] Jews who used the Greek translation of the Old Testament. For them to use a self-designation that was common in the Old Testament for the people of God reveals their understanding of the continuity that links the Old and New Testaments. The early Christians understood themselves as the people of the God who had revealed Himself in the Old Testament (Hebrews 1:1-2 Hebrews 1:1-2  God, who at sundry times and in divers manners spoke in time past to the fathers by the prophets,
 Has in these last days spoken to us by his Son, whom he has appointed heir of all things, by whom also he made the worlds;
American King James Version×), as the true children of Israel (Romans 2:28-29 Romans 2:28-29  For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh:  But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.
American King James Version×) with Abraham as their father (Romans 4:1-25 Romans 4:1-25  What shall we say then that Abraham our father, as pertaining to the flesh, has found?  For if Abraham were justified by works, he has whereof to glory; but not before God.  For what said the scripture? Abraham believed God, and it was counted to him for righteousness.  Now to him that works is the reward not reckoned of grace, but of debt.  But to him that works not, but believes on him that justifies the ungodly, his faith is counted for righteousness.  Even as David also describes the blessedness of the man, to whom God imputes righteousness without works,  Saying, Blessed are they whose iniquities are forgiven, and whose sins are covered.  Blessed is the man to whom the Lord will not impute sin.  Comes this blessedness then on the circumcision only, or on the uncircumcision also? for we say that faith was reckoned to Abraham for righteousness.  How was it then reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision.  And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed to them also:  And the father of circumcision to them who are not of the circumcision only, but who also walk in the steps of that faith of our father Abraham, which he had being yet uncircumcised.  For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith.  For if they which are of the law be heirs, faith is made void, and the promise made of none effect:  Because the law works wrath: for where no law is, there is no transgression.  Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all,  (As it is written, I have made you a father of many nations,) before him whom he believed, even God, who vivifies the dead, and calls those things which be not as though they were.  Who against hope believed in hope, that he might become the father of many nations, according to that which was spoken, So shall your seed be.  And being not weak in faith, he considered not his own body now dead, when he was about an hundred years old, neither yet the deadness of Sarah's womb:  He staggered not at the promise of God through unbelief; but was strong in faith, giving glory to God;  And being fully persuaded that, what he had promised, he was able also to perform.  And therefore it was imputed to him for righteousness.  Now it was not written for his sake alone, that it was imputed to him;  But for us also, to whom it shall be imputed, if we believe on him that raised up Jesus our Lord from the dead;  Who was delivered for our offenses, and was raised again for our justification.
American King James Version×), and as the people of the New Covenant prophesied in the Old Testament (Hebrews 8:1-13 Hebrews 8:1-13  Now of the things which we have spoken this is the sum: We have such an high priest, who is set on the right hand of the throne of the Majesty in the heavens;  A minister of the sanctuary, and of the true tabernacle, which the Lord pitched, and not man.  For every high priest is ordained to offer gifts and sacrifices: why it is of necessity that this man have somewhat also to offer.  For if he were on earth, he should not be a priest, seeing that there are priests that offer gifts according to the law:  Who serve to the example and shadow of heavenly things, as Moses was admonished of God when he was about to make the tabernacle: for, See, said he, that you make all things according to the pattern showed to you in the mount.  But now has he obtained a more excellent ministry, by how much also he is the mediator of a better covenant, which was established on better promises.  For if that first covenant had been faultless, then should no place have been sought for the second.  For finding fault with them, he said, Behold, the days come, said the Lord, when I will make a new covenant with the house of Israel and with the house of Judah:  Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, said the Lord.  For this is the covenant that I will make with the house of Israel after those days, said the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people:  And they shall not teach every man his neighbor, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest.  For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more.  In that he said, A new covenant, he has made the first old. Now that which decays and waxes old is ready to vanish away.
American King James Version×).
"As a consequence of this broad background of meaning in the Greek and Old Testament worlds, the term 'church' is used in the New Testament of a local congregation of called-out Christians, such as the 'church of God which is at Corinth' (1 Corinthians 1:2 1 Corinthians 1:2To the church of God which is at Corinth, to them that are sanctified in Christ Jesus, called to be saints, with all that in every place call on the name of Jesus Christ our Lord, both their's and our's:
American King James Version×), and also of the entire people of God, such as in the affirmation that Christ is 'the head over all things to the church, which is his body' (Ephesians 1:22-23 Ephesians 1:22-23  And has put all things under his feet, and gave him to be the head over all things to the church,  Which is his body, the fullness of him that fills all in all.
American King James Version×)."