[Tax-chick]

St Paul the Apostle defined church as a community of believers.

Citation, please?

[bobjam]

I Corinthians 1:2
"To the Church of God that is in Corinth; to those who are sanctified in Christ Jesus, called to be saints, together with all those who in every place call on the Name of our Lord Jesus Christ, both their Lord and ours."

St Paul is stating that all those who are called to be saints and are sanctified in the Lord are members of the Church.

I Corinthians 12:13
"For in the one Spirit we were all baptized into one body"

This is typical of St Paul. Throughout his epistles, he emphasizes that those who are baptized and are being sanctified are part of the Body of Christ, which is his Church. So long as they have the Holy Spirit and a functioning community of leaders, teachers, healers etc., they are a valid church.

[MeanWestTexan]

Not the exact cite you are looking for, but an even Higher authority than Paul on the general topic:

"For where two or three come together in my name, there am I with them." Matt 18:20

Effectively a church with the Boss there; I recall St. Paul saying much as represented by the other poster, but I do not know where.