How do the sacraments become effectual means of salvation?
The sacraments become effectual means of salvation, not from any virtue in them, or in him that doth administer them; but only by the blessing of Christ, and the working of his Spirit in them that by faith receive them1.
Scripture proofs
The Westminster Assembly's proof texts (KJV). The bracketed numbers in the answer above mark the clause each set of references supports; expand a row to read the verses.
1 1 Cor. 3:7; cf. I Cor. 1:12-17
So then neither is he that planteth any thing, neither he that watereth; but God that giveth the increase.
1 Cor. 1:12 Now this I say, that every one of you saith, I am of Paul; and I of Apollos; and I of Cephas; and I of Christ.
1 Cor. 1:13 Is Christ divided? was Paul crucified for you? or were ye baptized in the name of Paul?
1 Cor. 1:14 I thank God that I baptized none of you, but Crispus and Gaius;
1 Cor. 1:15 Lest any should say that I had baptized in mine own name.
1 Cor. 1:16 And I baptized also the household of Stephanas: besides, I know not whether I baptized any other.
1 Cor. 1:17 For Christ sent me not to baptize, but to preach the gospel: not with wisdom of words, lest the cross of Christ should be made of none effect.
This doctrine across the Standards
Where the Confession and the Westminster Larger Catechism treat the same matter, so the question can be studied alongside its parallel statements.
In the Westminster Confession
VII.6 Under the gospel, when Christ the substance was exhibited, the ordinances in which this covenant is dispensed are …
Under the gospel, when Christ the substance was exhibited, the ordinances in which this covenant is dispensed are the preaching of the Word and the administration of the sacraments of Baptism and the Lord's Supper; which, though fewer in number, and administered with more simplicity and less outward glory, yet in them it is held forth in more fulness, evidence, and spiritual efficacy, to all nations, both Jews and Gentiles; and is called the New Testament. There are not, therefore, two covenants of grace differing in substance, but one and the same under various dispensations.
→ VII.6 · Of God's Covenant with ManXXVII.1 Sacraments are holy signs and seals of the covenant of grace, immediately instituted by God, to represent Christ …
Sacraments are holy signs and seals of the covenant of grace, immediately instituted by God, to represent Christ and his benefits, and to confirm our interest in him: as also to put a visible difference between those that belong unto the Church and the rest of the world; and solemnly to engage them to the service of God in Christ, according to his Word.
→ XXVII.1 · Of the SacramentsXXVII.2 There is in every sacrament a spiritual relation or sacramental union, between the sign and the thing signified; …
There is in every sacrament a spiritual relation or sacramental union, between the sign and the thing signified; whence it comes to pass that the names and the effects of the one are attributed to the other.
→ XXVII.2 · Of the SacramentsXXVII.3 The grace which is exhibited in or by the sacraments, rightly used, is not conferred by any power …
The grace which is exhibited in or by the sacraments, rightly used, is not conferred by any power in them; neither doth the efficacy of a sacrament depend upon the piety or intention of him that doth administer it, but upon the work of the Spirit, and the word of institution, which contains, together with a precept authorizing the use thereof, a promise of benefit to worthy receivers.
→ XXVII.3 · Of the SacramentsXXVII.4 There be only two sacraments ordained by Christ our Lord in the Gospel, that is to say, Baptism …
There be only two sacraments ordained by Christ our Lord in the Gospel, that is to say, Baptism and the Supper of the Lord: neither of which may be dispensed by any but by a minister of the Word lawfully ordained.
→ XXVII.4 · Of the SacramentsXXVII.5 The sacraments of the Old Testament, in regard of the spiritual things thereby signified and exhibited, were, for …
The sacraments of the Old Testament, in regard of the spiritual things thereby signified and exhibited, were, for substance, the same with those of the New.
→ XXVII.5 · Of the SacramentsIn the Westminster Larger Catechism
Q.161 How do the sacraments become effectual means of salvation?
The sacraments become effectual means of salvation, not by any power in themselves, or any virtue derived from the piety or intention of him by whom they are administered, but only by the working of the Holy Ghost, and the blessing of Christ, by whom they are instituted.
→ Open Q.161Q.162 What is a sacrament?
A sacrament is a holy ordinance instituted by Christ in his church, to signify, seal, and exhibit unto those that are within the covenant of grace, the benefits of his mediation; to strengthen and increase their faith, and all other graces; to oblige them to obedience; to testify and cherish their love and communion one with another; and to distinguish them from those that are without.
→ Open Q.162Q.163 What are the parts of a sacrament?
The parts of a sacrament are two; the one an outward and sensible sign, used according to Christ's own appointment; the other an inward and spiritual grace thereby signified.
→ Open Q.163Q.164 How many sacraments hath Christ instituted in his church under the New Testament?
Under the New Testament Christ hath instituted in his church only two sacraments, Baptism and the Lord's supper.
→ Open Q.164