There are some who unconsciously or consciously want to belong to the church, to such an extent that they cannot imagine not belonging to it, and who are in a state of such doubt about the basic assertion that Jesus is the Christ and its implications that they are on the verge of separating themselves from the church, at least inwardly. ... They belong to the church but they doubt whether they belong. For them it must be said that the criterion of one's belonging to a church and through it to the Spiritual Community is a serious desire, conscious or unconscious, to participate in the life of a group which is based on the New Being as it has appeared in Jesus as the Christ. Such an interpretation can help people whose consciences are troubled by misgivings about the whole set of symbols to which the subject themselves in thought, devotion and action.Its a central feature in Tillich's theological thinking that doubt is something that is accepted. Tillich applies the notion of grace to it, claiming that it is not only the sinner that is accepted by God, but also the doubter. In this text I think Tillich shows a deep understanding of the nature of the doubt many people experience in the Christian faith. It is more seldom the idea of belonging to the church that is the problem, but the way faith is expressed. Tillich says that a desire, even unconscious, to be part of the church is enough to be part of it, even if one has difficulties with expressions of the doctrine and so on. He is able to assert this because he is aware of the transient nature of all formulations of the dogma. The Spiritual Community is not based on a common language game, it is based on a longing for sharing love. That longing is the criterion of membership.
Systematic Theology III, 175.
(also see this post for some of my own thoughts on the subject.)