Neither divorce nor remarriage is in itself the unforgivable sin any more than murder, stealing, lying or coveting. "All sins will be forgiven the sons of men." God is faithful and just to forgive—he will honor the worth of his Son's sacrifice for all who confess their sin and bank their hope on the saving work of Christ.
Forgiveness is NOT unconditional. It is conditional. This does not mean it can be earned. It means forgiveness is given to those who truly trust Christ. Trust is not an act by which anything can be earned. It calls attention to the worth of God's grace, not the worth of our action. But trust is not mere intellectual assent to Biblical facts. It involves hearty affirmation of the will of Christ. Therefore trusting Christ involves confessing sin as sin and taking up arms against it.
Therefore the ultimate form of church discipline (excommunication) is never a simple response to past sin. It is always a response to sin that a person continues to affirm or practice. No past sin that is renounced, confessed and forsaken is a ground of church discipline.
Therefore marital sin is in the same category as lying and killing and stealing when it comes to church discipline and church membership. If someone has lied, killed, stolen, or illegitimately divorced, the issue is not, can they be forgiven? The issue is do they admit that what they did was sin? Do they renounce it? And do they do what they can to make it right?
If a person in the church was known to affirm lying, killing or stealing as appropriate behavior for a Christian, that person would be liable to the discipline of the church. Not because they have lied, killed or stolen in the past and cannot be forgiven, but because they go on affirming NOW that sin is not sin.
Or if a person was openly planning to lie, kill or steal with a view to receiving (cheap!) forgiveness afterward, that person too would be liable to church discipline.
In all these ways illegitimate divorce and remarriage are NOT in a class by themselves. They are not the unforgivable sin. When it comes to church discipline and church membership they should be treated the same way other public sins are treated.