Calvin held to the ancient church doctrine of the ever-virginity of the Blessed Virgin, and your (unstated) view that this is not so is a contradiction.
The doctrinal standards of the OPC are the Westminster Confession of Faith and Catechisms as adopted by the OPC in 1936. It is true that our standards do not affirm the perpetual virginity of Mary, the mother of our Lord. Neither do the standards deny it: they do not address the question. The Confession of Faith mentions Mary in 8.2:
The Son of God, the second person in the Trinity, being very and eternal God, of one substance and equal with the Father, did, when the fullness of time was come, take upon him man’s nature, with all the essential properties, and common infirmities thereof, yet without sin; being conceived by the power of the Holy Ghost, in the womb of the virgin Mary, of her substance. So that two whole, perfect, and distinct natures, the Godhead and the manhood, were inseparably joined together in one person, without conversion, composition, or confusion. Which person is very God, and very man, yet one Christ, the only Mediator between God and man.
The Catechisms likewise refer to her, although one additional reference in the Larger Catechism might be said to apply to her (among others), affirming that Christ is the sole mediator:
Q. 181. Why are we to pray in the name of Christ?
A. The sinfulness of man, and his distance from God by reason thereof, being so great, as that we can have no access into his presence without a mediator; and there being none in heaven or earth appointed to, or fit for, that glorious work but Christ alone, we are to pray in no other name but his only.
As for Calvin, his commentary on Matthew 1:25 and Luke 1:34 would not seem to support the contention that he affirmed Mary’s perpetual virginity. At the most, he appears to be agnostic on the question, which is to say that he neither clearly affirms nor denies it. In his commentary on James, Calvin notes the near unanimous opinion of the ancients that James was “a relative” of Jesus, without detailing the nature of that kinship. It is also the case, however, that Calvin decidedly rejects the “erroneous doctrines of the intercession of the saints” (Institutes of the Christian Religion, III.xx.21–27), which has impact on Mariological matters.
I trust that this proves helpful.
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