Simply put, a Church is a gathering of local professing believers (2 Corinthians 8:1; 11:28). This is what makes up the "visible church" or the physical gathering of people claiming to be Christian. The word church is a translation of the Greek word ekklesia, meaning “a called–out assembly.” And the notion that any one denomination is the "true church" is a deceptively false claim.
BUT, not everyone who attends church is actually a born-gain saved believer, even if they open profess it. They do not make up the "invisible church" or the spiritually alive, born-again, repentant followers of Christ Jesus. This is only whom God sees, his actual adopted Children and the Bride of Christ (Revelation 21:2, 9; 22:17). Christ Jesus is the head of the "invisible" Church (Colossians 1:18; Ephesians 3:21), and over all those who are actually part of the body of Christ (Ephesians 4:12; 1 Corinthians 12:27). THE Church is made up of an unnamed, unspecified group of individuals, known by God (Philippians 3:6; 1 Timothy 3:5).
Atheists have and can claim to be Christian. Atheists can also run out and get baptized by any visible church willing to baptize them. But all this is solely based on the will of man (speech and action) and not compelled and caused by the will of God.
Jesus reveals to us this: "On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’" Matthew 7:22-23. They may have claimed to know Jesus and claimed to have done all these "good" deeds in his name, but Jesus doesn't know them.
A 2010 study by Daniel Dennett and Linda LaScola called “Preachers Who Are Not Believers.” Dennett and LaScola’s work chronicles five different preachers who over time were presented with and accepted heretical teachings about Christianity and now have completely fallen away from the faith. These pastors are either pantheists or clandestine atheists. One of the most disturbing truths highlighted in the study is that these preachers maintain their position as pastors of Christian churches with their congregations being unaware of their leader’s true spiritual state.
Thus, the physical Church you see and attend, can, maybe, and are riddled with people that Jesus doesn't even know as his own. Jude gives us a heads up: “For certain persons have crept in unnoticed, those who were long beforehand marked out for this condemnation, ungodly persons who turn the grace of our God into licentiousness and deny our only Master and Lord, Jesus Christ” (Jude 1:4). Paul sees what Jude is seeing in the Church, and states: “For such men are false apostles, deceitful workers, disguising themselves as apostles of Christ. No wonder, for even Satan disguises himself as an angel of light” (2 Corinthians 11:13–14). The Holy Spirit, speaking through Paul states: "They profess to know God, but by their deeds they deny Him, being detestable and disobedient and worthless for any good deed” (Titus 1:15–16). The Apostle John states this: “They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us” (1 John 2:19).
All this to say that there ARE and WILL BE false Christians who attend church and act religious. Therefore its not their presence and attendance that should detour anyone from going to church. Every single saved Christian is also still a sinner, though they are maturing and being sanctified, they are still imperfect people until they day they are glorified before the Lord after leaving this world. The faithfulness of a THE Church is not measured by the flimsy faithfulness of man, but by Jesus Christ, who sits at the head of THE Church and by The Holy Spirit to moves invisibly through THE Church.
Therefore, what a TRUE CHURCH is, is not based on the sinners that make it up. Thus, a true church is based the TRUTHS it proclaims.
"6 I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel— 7 not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. 8 But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed." Galatians 1:6-8
"But I am afraid that as the serpent deceived Eve by his cunning, your thoughts will be led astray from a sincere and pure devotion to Christ. For if someone comes and proclaims another Jesus than the one we proclaimed, or if you receive a different spirit from the one you received, or if you accept a different gospel from the one you accepted, you put up with it readily enough." 2 Corinthians 11:3-4
Another (different) Jesus isn't the real Jesus
A different, distorted Gospel - isn't the gospel that saves
A different spirit - proclaims a different, distorted gospel
Even if a pastor, elder, Prophet, Apostle, and or Angel from Heaven - they are of a different spirit and not of God.
Another, different Jesus is one that is NOT found or vastly different than the one in the synoptic gospels, Matthew, Mark, Luke, and John; different from the one prophesized about in the Old Testament; and different from the Jesus articulated in the Holy Spirit inspired Epistles.
So, which is the true Jesus? "...the one we preached to you" and "another Jesus than the one we proclaimed" The "we" are the Apostles and Prophets who wrote and or directed the writing of Holy Scripture under the direct inspiration of The Holy Spirit.
These are historic orthodox teachings of the Christian faith from scripture and throughout history, keeping in line with scripture. As heresies and "different gospels" and "another Jesus" are taught, the Church, through Holy Scripture and the Holy Spirit, articulate what God's Word has revealed regarding what the heresy is alleging down through history.
Monotheistic Trinitarianism - 1 God being/essence in 3 distinct co-equal persons: God The Father, God the Son, and God the Holy Spirit.
Hypostatic Union of Jesus Christ - Jesus, the son of Man, fully Man; Jesus, The Son of God, fully God.
The Sin Nature of all people - All people are sinners a conception.
Penal Substitutionary Limited Atonement - Perfect justice and propitiation - Justification.
The Just penalty for sin, Jesus' physical death, physical resurrection on the 3rd day, and actual and physical appetences. - The true Gospel
Jesus' actual and physical ascension and physical future return.
Unconditional Election and regeneration - Salvation by grace
The 3rd person of the Triune God, The Holy Spirit; and his work in and through the church
Holy Scripture is God's word and is the authority of all spiritual matters.
Saving faith proceeds good works, and it's faith that compels works; thus, works will be desired and evident in an already saved believer, simply because they are saved
A false church must first try and discredit Holy Scripture. Once it has deceived it's followers to doubt what Holy Scripture teaches, then it can usher in a different, distorted Jesus and distorted gospel.
It usually takes more minor issues, and secondary teachings and changes those first. Usually it's what THEY feel are "correct" teachings of "allowable behaviors" and to justify this change, they also change or "correct" who the authority is to make these changes to the new allowable behaviors. This is why churches who justify female pastors will also, usually, excuse fornicative behaviors, such as homosexuality; and tend to act and teach on things more worldly than what is explicit in Scripture. To justify these two, they would also need to re-interpret or "correct" what the church has taught for thousands of years from the 1st century church.
A false church asserts its OWN authority over how they want their followers to understand the Bible. Sometimes, they will even deceptively publish their own bible translation that changes the things they want to discredit.
Islam - An angel gave Muhammad a different gospel and described a vastly different Jesus, who then wrote their own "holy book"
Mormonism - An angel gave Joseph Smith a different gospel and described a vastly different Jesus, who then wrote their own "holy book"
Jehovah's Witnesses - Watch Tower Society teaches a vastly different gospel and a different Jesus, then tells followers how to read their OWN poorly translated Bible version.
Seventh Day Adventist - Founders had visions from "God," despite some visions became untrue, presenting a different gospel message.
This is when a seemingly healthy church leaves Biblical Doctrine and slips away from revealed truth to start teaching something explicitly contrary to what the Prophets and Apostles proclaimed and preached as revealed in Holy Scripture
Presbyterian Church USA, The Episcopal Church, The Evangelical Lutheran Church, The United Methodist Church, and The United Church of Christ - left the doctrine taught in Holy Scripture, Old and New Testaments, for thousands of years; about Woman Pastors, Marriage, and Fornication by proclaiming support for and justifying lesbian female pastors and attendees; directly leading their parishioners into not being able to inherit eternal life (1 Cor 6:9).
Joseph Smith as affiliated with the Methodists was kicked out and left due to his unorthodox and heretical views. He, his wife, and other followers left the orthodox Christan faith and started their own.
Charles Taze Russell left the Presbyterian Church, influenced by Millerite Adventist, to start his own "ministry" and "mission" which transformed into the Jehovah's Witnesses. Where he incorporated the heresy of Arianism into his teachings.