Q: How do the sacraments become effectual means of salvation?
A: 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 them.

1 Peter 3:21. The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ.
Acts 8:13, 23. Then Simon himself believed also: and when he was baptized, he continued with Philip, and wondered, beholding the miracles and signs which were done. . . For I perceive that thou art in the gall of bitterness, and in the bond of iniquity.
1 Corinthians 3:7. So then neither is he that planteth any thing, neither he that watereth; but God that giveth the increase.
1 Corinthians 6:11. And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God.
1 Corinthians 12:13. For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit.

Westminster Project - Matthew Henry's Scripture Catechism based on the WSC