G.I. Williamson is a former OPC minister and author of various study guides of Reformed confessions. Here are some goodies from his Westminster Confession study guide.
On Liberty (XX):
Examples of rules which are contrary to the Word of God are prohibitions requiring total abstinence from the use of certain material things. … However, in not one case is it possible to show that such abstinence is required of God [Rom 14:14, 20]… It is true, of course that once a person has allowed his conscience to be bound by such a (false) rule, he cannot partake of the forbidden thing without sinning. [Rom 14:14, 20, 23]. … It is never right to do what we believe to be wrong, even when we believe a thing to be wrong without good reason. But even if a person faithfully obeys his conscience and scrupulously observes a rule forbidding the use of a material thing, he is still guilty of sin. He is guilty of the sin of allowing someone other than God to impose a rule upon his conscience. To this it is objected that without such rules…the only possible result will be “all-out intemperance.” … We shall only say here that it is extremely dishonoring to the Spirit of God to maintain such an objection. For this objection is tantamount to saying that a man-made rule will keep a Christian from sin better than the Holy Spirit who dwells in him.
On Divorce (XXIV):
…Christians, by reason of the sinful propensities remaining in them, are apt to invent arguments to justify divorce for other than [the two Biblical reasons, adultery and abandonment]. For example, when fidelity to the marriage requirements entails heartache and suffering, many a Christian has tried to justify separation and divorce… Others have secured divorces from spouses that are imprisoned or hospitalized. But… the path of obedience is often the way of self-denial and the bearing of reproach and suffering for the glory of God.
On Baptism (XXVIII):
We must not limit the efficacy of baptism to the moment of administration. … [W]e cite the case of Jacob and Esau. (1) Both were given the sacrament by divine commandment. (2) Esau never received the grace of which he had the sign and seal. And (3) Jacob did not experience the efficacy of the sacrament until his conversion many years later… The purpose of baptism is not to effect union with Christ but rather to confirm and testify such. …[B]aptism testifies that God gives union with Christ to whom he will, as he will, and when he will. … Baptism, like circumcision, may have no effect on some people. But infant baptism, like infant circumcision, does have a profound effect upon some who are converted long after they are baptized. … For one cannot experience the efficacy of baptism prior to baptism… p. 215
And this on the use of grape juice (XXIX):
[T]hose who ordinarily use leavened bread and grape juice out of mere convenience we will not condemn. But if the decision to use grape juice instead of wine is based on the influence of the Temperance Movement, we must regard this as seriously unbliblical. It is a false doctrine, a legacy from the ancient Gnostics, to locate sin or evil in material things. The cause of the sin of drunkenness was located by Christ in man’s depraved heart (Mark 7:14-28), not in wine.