The OPC does not have a declared position on whether grape juice or fermented wine is served in the Lord’s Supper. Some congregations use fermented wine, though I would judge that the majority of churches are content to serve grape juice.

The common word for wine in the New Testament is oinos (οἶνος). As we know, in Bible times there was no means of keeping grape juice unfermented, so grape juice inevitably turned into wine as we know it today. There was another Greek word, gleukos (γλεῦκος), which means sweet wine, or new wine. It occurs only once in the New Testament (Acts 2:13).

In Matthew 26:29 and Mark 14:25, our Lord refers to the sacramental cup as “the fruit of the vine.” That could be understood as allowing the use of either fresh grape juice or fermented wine, but, to my knowledge, the matter has never come before any General Assembly. Apparently the choice is left to sessional decision.

As to why the predominant practice is on the side of grape juice, I can only suggest that tradition is the reason.

Can a biblical case be made for the exclusive use of wine as we know it today? I doubt that it can be sustained. Gleukos appears but once in the NT. That is not strong enough to justify a clear distinction between oinos as uniformly fermented grape juice and gleukos as fresh or sweet wine.

I hope that answers your question, but feel free to write again if you have further questions.