The Greatest Weapon

If we as the Church Militant are fighting a spiritual battle, then what is our greatest weapon?
Well, while you probably could come up with lots of pretty powerful weapons the Church gives us, the title "greatest weapon" has already been claimed... by the Rosary.
The Rosary is a prayer dedicated to Our Lady, made up of One Apostle's Creed, six Our Fathers, 53 Hail Mary's, 6 Glory Be's and one Hail, Hail Queen. It has been a prayer of the universal church for centuries.
But that is just the tip of the iceberg.
The Rosary combines mental and vocal prayer. It combines intercessory prayer with prayers of praise and thanksgiving. You worship the Trinity, you profess your faith, you honor God's beloved mother, you put yourself on God's side and make some powerful friends up in Heaven. By saying one Rosary, you have pretty much covered your bases.
To start from the top, 'intercession' means to ask in the place of. So we ask Our Lady to go to God in our stead to present Him with our petitions. It’s like asking your cute little sister to go ask Dad to for an ice cream cone. Your dad just can’t say no to her. So it is with Our Lady, who was so beloved of God that He can’t say no to her. Anything we ask of God through Our Lady He will grant us. She is His mother, chosen from all the women throughout the history of the world to bear His son. She cared for Him, imitated Him, and suffered with Him. She stood at the foot of the cross when He and the truth He taught were at their ugliest. And so, before starting the rosary, we say our petitions. We list the things we want from God, thanking Him for what He has already done for us, and asking Him to make us saints, asking our Lady to remind Her Son of the promises He made to save us from sin and death. Then we pray the prayers. We start with the Apostle's Creed, which goes through what we believe as Catholics. If you ever need a quick way to answer anyone who asks you what you believe in, say the Apostles creed. The Our Father comes directly from the Bible, from the Sermon on the Mount when the apostles asked Jesus how to pray. He teaches them the Our Father. In it, we are taught to speak to God as a father. We praise His name, and we pray that His will is done on earth. Then we ask that He help us every day and give us what we need. We ask for forgiveness, and we promise to forgive. We ask for strength against the temptations of Satan. The Hail Mary comes from the words of the the angel when he came to ask Mary to be the mother of God and from the words of Mary’s cousin St Elizabeth. We repeat these words to give her and her Son honor. Then we ask her to care for us every day of our lives. We put ourselves into her care.
Each set of one Our Father and ten Hail Mary’s, called a decade, is dedicated to a certain event in the lives of Jesus and Mary. We are supposed to think about that event while we pray the prayers with our mouths. When we announce the Crucifixion, we are supposed to think about how Jesus loved us so much that He died for our sins, and how much every sin we commit hurts Him. Prayers in our mind are called mental prayers or meditation, and prayers with our mouths are called vocal prayers. The rosary combines these two types of prayer. This is perfect for people just starting in the faith, for they can learn how to pray by saying the words. As you grow in your spiritual life, you can begin to picture each scene and begin to understand the mysteries of each decade. There are five decades in one mystery. It only takes about 15-20 minutes to say one mystery (a TV episode is longer!) and there are four different mysteries. They are called mysteries because they are hard to understand. The Joyful Mysteries cover the conception, birth and childhood of Jesus. The Luminous mysteries come next and go over the public ministry of Jesus and the preaching of the word. The Sorrowful mysteries cover His Passion and Death. The final mystery is called Glorious because the decades cover the Resurrection of Jesus from the dead and events in the church after He ascends.
You should pray all four mysteries every day, since the four mysteries together equal one rosary, but usually people just say the mystery that is assigned to the day. Saturday and Monday are Joyful mysteries, Tuesday and Friday are Sorrowful and Thursdays are Luminous. Sundays and Wednesdays are glorious.
If you need more reasons to actually say the rosary, here some of the 15 promises made by Our Lady to Blessed Alan de le Roche about the Rosary:

To all those who shall pray my Rosary devoutly, I promise my special protection and great graces.

The Rosary will be a very powerful armor against hell; it will destroy vice, deliver from sin and dispel heresy.

The rosary will make virtue and good works flourish, and will obtain for souls the most abundant divine mercies. It will draw the hearts of men from the love of the world and its vanities, and will lift them to the desire of eternal things. Oh, that souls would sanctify themselves by this means.

Those who trust themselves to me through the Rosary will not perish.

Whoever recites my Rosary devoutly reflecting on the mysteries, shall never be overwhelmed by misfortune. He will not experience the anger of God nor will he perish by an unprovided death. The sinner will be converted; the just will persevere in grace and merit eternal life.

Those who are faithful to recite my Rosary shall have during their life and at their death the light of God and the plenitude of His graces and will share in the merits of the blessed.

I will deliver promptly from purgatory souls devoted to my Rosary.

True children of my Rosary will enjoy great glory in heaven.

What you shall ask through my Rosary you shall obtain.