Q: How is the word made effectual to salvation?
A: The Spirit of God makes the reading, and especially the preaching of the word an effectual means of convincing and converting sinners, and of building them up in holiness and comfort through faith unto salvation.

Psalm 19:7. The law of the Lord is perfect, converting the soul: the testimony of the Lordis sure, making wise the simple.
Psalm 119:130. The entrance of thy words giveth light; it giveth understanding unto the simple.
Hebrews 4:12. For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.
1 Thessalonians 1:6. And ye became followers of us, and of the Lord, having received the word in much affliction, with joy of the Holy Ghost.
Romans 1:16. For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
Romans 16:25. Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began.
Acts 20:32. And now, brethren, I commend you to God, and to the word of his grace, which is able to build you up, and to give you an inheritance among all them which are sanctified.

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