From The Great Means of Salvation and of Perfection by St. Alphonsus Liguori, 2.2:
God knows the great good which it does us to be obliged to pray, and therefore permits us [as we have already shown in the previous chapter] to be assaulted by our enemies, in order that we may ask Him for the help which He offers and promises to us. But as He is pleased when we run to Him in our dangers, so is He displeased when He sees us neglectful of prayer. "As the king," says St. Bonaventure, "would think it faithlessness in an officer, when his post was attacked, not to ask him for reinforcements, he would be reputed a traitor if he did not request help from the king;" so God thinks Himself betrayed by the man who, when he finds himself surrounded by temptations, does not run to Him for assistance. For He desires to help us; and only waits to be asked, and then gives abundant succour.
This is strikingly shown by Isaias, when, on God's part, he told the king Achaz to ask some sign to assure himself of God's readiness to help him: "Ask thee a sign of the Lord Thy God." [Is. 7: 2] The faithless king answered: "I will not ask, and I will not tempt the Lord;" for he trusted in his own power to overcome his enemies without God's aid. And for this the Prophet reproved him: "Hear, therefore, O house of David; is it a small thing for you to be grievous to men, that you are grievous to my God also?" because that man is grievous and offensive to God who will not ask Him for the graces which He offers. "Come to Me, all you that labor and are burdened, and I will refresh you." [Matt. 11: 28] "My poor children," says our Savior, "though you find yourselves assailed by enemies, and oppressed with the weight of your sins, do not lose heart but have recourse to Me in prayer, and I will give you strength to resist, and I will give you a remedy for all your disasters." In another place he says, by the mouth of Isaias, "Come and accuse Me, saith the Lord; if your sins be as scarlet, they shall be made white as snow." [Is. 1: 18] O men, come to Me; though your consciences are horribly defiled, yet come; I even give you leave to reproach Me [so to speak], if after you had recourse to Me, I do not give you grace to become white as snow. What is prayer? It is, as St. Chrysostom says, "the anchor of those tossed on the sea, the treasure of the poor, the cure of diseases, the safeguard of health." It is a secure anchor for him who is in peril of shipwreck; it is a treasury of immense wealth for him who is poor; it is a most efficacious medicine for him who is sick; and it is a certain preservative for him who would keep himself well.
What does prayer effect? Let us hear St. Laurence Justinian: "It pleases God, it gets what it asks, it overcomes enemies, it changes men." It appeases the wrath of God, Who pardons all who pray with humility. It obtains every grace that is asked for; it vanquishes all the strength of the tempter, and it changes men from blind into seeing, from weak into strong, from sinners into Saints. Let him who wants light ask it of God, and it shall be given. As soon as I had recourse to God, says Solomon, He granted me wisdom: "I called upon God, and the Spirit of wisdom came to me." [Wisd. 7: 7] Let him who wants fortitude ask it of God, and it shall be given. As soon as I opened my mouth to pray, says David, I received help from God: "I opened my mouth, and drew in the Spirit." [Ps. 118: 131] And how in the world did the Martyrs obtain strength to resist tyrants, except by prayer, which gave them force to overcome dangers and death? "He who uses this great weapon," says St. Chrysostom, "knows not death, leaves the earth, enters Heaven, lives with God." He falls not into sin; he loses affection for the earth; he makes his abode in Heaven; and begins, even in this life, to enjoy the conversation of God. How then can you disquiet such a man by saying: "How do you know that you are written in the book of life?" How do you know whether God will give you efficacious grace and the gift of perseverance? "Be nothing solicitous," says St. Paul, "but in everything by prayer and supplication, with thanksgiving, let your petitions be known unto God." [Phil. 4: 6] What is the use, says the Apostle, of agitating yourselves with these miseries and fears? Drive from you all these cares, which are of no use but to lessen your confidence, and to make you more tepid and slothful in walking along the way of salvation.
Pray and seek always, and make your prayers sound in God's ears, and thank Him for having promised to give you the gifts which you desire whenever you ask for them, namely efficacious grace, perseverance, salvation, and everything that you desire. The Lord has given us our post in the battle against powerful foes; but He is faithful in His promises, and will never allow us to be assaulted more violently than we can resist: "God is faithful, Who will not suffer you to be tempted above that which ye are able." [1 Cor. 10: 13]
He is faithful, since He instantly succours the man who invokes Him. The learned Cardinal Gatti writes, that God has bound Himself not only to give us grace precisely balancing the temptation that assails us, but that He is obliged, when we are tempted, and have recourse to Him, to afford us, by means of that grace which is kept ready for and offered to all, sufficient strength for us actually to resist the temptation. "God is bound, when we are tempted, and fly to His protection, to give us by the grace prepared and offered to all such strength as will not only put us in the way of being able to resist, but will also make us resist; 'for we can do all things in Him who strengthens us' by His grace, if we humbly ask for it." [De Grat., q. 2, d. 5, Sect. 3]
We can do all things with God's help, which is granted to every one who humbly seeks it; so that we have no excuse when we allow ourselves to be overcome by a temptation. We are conquered solely by our own fault, because we would not pray. By prayer all the snares and power of the devil are easily overcome. "By prayer all hurtful things are chased away," says St. Augustine.
The whole book is available here: http://www.catholictreasury.info/books/prayer/index.php