Consider the following scenario. A person starts attending worship services and appreciates the preaching of God's word. Members of the church give the visitor a warm welcome. Consequently, he comes back often and learns a lot about the Bible in a friendly atmosphere. After a while, he is regularly attending Bible classes and worship services. Wanting to support the church and its work, he commits to giving a percentage of his income every Sunday. Soon, this good man even volunteers to help with repairs around the building.
Eventually this person no longer considers himself a visitor. Rather, he thinks he is a member of the congregation since he has invested a lot of time, money, and effort in the congregation. However, does this person's regular attendance, contributions, and work make him a member of the church? No.
How then does a person become a member of the Lord's church? If a person does what God says is necessary to become a member, then he is a member. Otherwise, he is not a member no matter what he or others may think. When a person obeys the command to repent and be baptized for the remission of his sins, he is saved (cf. Acts 2:38). Luke recorded that "the Lord added to the church daily those who were being saved" (Acts 2:47). The church does not (and should not) vote on whether or not to accept a person as a member since it is the Lord who adds souls to the church.
To what church does He add them? Before He died, the Lord said he would build His church (cf. Matt. 16:18). After the Lord's resurrection and ascension, which church existed? The answer: the church which Jesus built (and it alone)! On Pentecost, souls were added to that church--the church of the Christ (cf. Rom. 16:16). There were no others yet in existence. Today, if a person likewise obeys the gospel, Jesus will add that soul to the Lord's church (i.e., His church) and not a man-made church.
Perhaps a modern day illustration will be helpful. Suppose a man wanted to join a sportsman's club so he could have a place for some target shooting. Before becoming a member of the club he is required to attend a training class to learn the club bylaws and to make sure he knows how to safely handle firearms. At the end of the sessions he must take a test to ensure he knows the range rules. Also, he must pay the membership dues. Then, and only then, he is admitted as a member of the club. One requirement for all members is participation in the work day to maintain and improve the facilities.
Before becoming a member this man could have attended every shooting event open to the public. He also could have contributed money to help with their expenses. Likewise, I am sure no one would have turned him away if he showed up on a work day to cut the grass or do repairs around the property. However, even if this sportsman faithfully attended every event, gave generously of his money, and labored mightily about the property, he still would only be a guest and not a member. Why? Is it because he was not good enough? No, it is because he had not complied with the club's rules for obtaining membership.
Will continued attendance make a visitor into a Christian? No. Faithful attendance, though commendable, does not have such transforming power. A fox may regularly attend the chicken coop but this does not make him a chicken one day!
Even the most upright and moral non-Christian is still a lost soul and not part of the body of Christ (e.g., Acts 10:1-6ff). The only way to become a member of the Lord's church is to obey the gospel. Doing this saves the soul and also adds one to the universal church. This is the only way to become a Christian (cf. Acts 4:12).