Now, that is a question which would really take a complete book to answer fully, but here are two main points to consider,

At one level, the church is the total number of God's people everywhere, When Paul said that Christ is 'head over everything for the church which is his body,' (Eph. 1:22,23) he was speaking not just of the Christians who belonged to a church in Ephesus but of everyone everywhere who believed in Christ, This is usually called the 'universal church'.

At a second level, Paul talked about 'the churches of God' (1 Cor, 11:16) and he wrote letters addressed to separate companies of God's people as in 1 Cor.1:2 and lThess,1:1,These are examples of 'local' churches - not the buildings. of course, but the groups of people whose lives had been changed by Christ, and who gathered together to worship God in particular places,

Please note - the New Testament knows nothing about a Christian belonging to the universal church while at the same time neglecting to become a member of a local church.