Martin Luther said and wrote many heretical things, but here are a mere five quotes, along with Biblical passages that disprove them.
1. “For reason is directly opposed to faith. This is why you must let reason go. It must be killed and buried in believers.” 
(Answer: Proverbs 14:15, Colossians 1:9, Colossians 4:5-6, 1 Thessalonians 5:19-22)
2. “..[A Christian] could not lose his salvation, however much he sinned, unless he refused to believe. For no sin can condemn him save unbelief alone.” 
(Answer: Hebrews 10:26-27)
3. “Be a sinner, and let your sins be strong, but let your trust in Christ be stronger, and rejoice in Christ who is the victor over sin, death, and the world. We will commit sins while we are here, for this life is not a place where justice resides. …No sin can separate us from Him, even if we were to kill or commit adultery thousands of times each day.” 
(Answer: Psalm 4:4, Matthew 5:48, Hebrews 10:26-27)
4. “It is good that we have such a man [Jesus Christ], because God in himself is cruel and bad.” 
(Answer: Psalm 51:1, Jeremiah 9:24, Jeremiah 31:3)
5. “I miss more than one thing in this book [Revelation, aka Apocalypse], and this makes me hold it to be neither apostolic nor prophetic…and [I] can nohow detect that the Holy Spirit produced it. …My spirit cannot fit itself into this book.” 
(Answer: Revelation 22:19)
1. Luther’s Sermon on Matthew 19:13-15. “What Luther Says,” 1:485-486 (entry 1440). [link]
3. Let Your Sins Be Strong: A Letter From Luther to Melanchthon. Translated by Erika Bullmann Flores. [link]
5. M. Reu, Luther’s German Bible: An Historical Presentation Together with a collection of Sources (Ohio: The Lutheran Book Concern, 1934) 174-175 [link]
Protestants claim that Confession (aka the Sacrament of Reconciliation) is unnecessary, but that claim totally contradicts the Word of God.
Kevin M. Tierney wrote at Catholic Lane:
When one repents of their sins in the Bible, it is always done to another individual.
The clearest case of this is with David after he commits adultery and arranges the murder of the woman’s husband. David only repents of his sin once God’s representative Nathan confronts him. (2 Samuel 12:1-13) David knew he had sinned grievously in his adultery, otherwise he would not have had Uriah killed to conceal his crime. Even knowing the extent of his guilt, he refused to repent. This speaks to the human psyche’s ability to rationalize away what they do so that it is no longer a sin. This is a skill humanity has nearly perfected in today’s age.
Another thing worth considering is how professing something vocally changes things. It is very easy to say something silently with no witnesses. It is something altogether different when you have to acknowledge your faults before another. One could say it becomes a far more serious endeavor when you are not only willing to renounce your sins, but renounce them forcefully out loud. The first step on the road to repentance requires you to renounce those sins. While it possible to fake such, it becomes far harder to do so. (It goes without saying that such a faked confession would be a sacrilege, and compound sin upon sin!)
Whenever I hear Protestants say that confessing sins to a priest is wrong, I am reminded of Luke 5:21, in which the Pharisees say that only God can forgive sins, and doubt Christ’s ability to do so. They are so blinded by their ideology that they can not recognize that God (Christ is God in human form, both fully human and fully divine) is before them.
Am I brazen enough to compare priests to Christ? In a way, yes, because priests serve in persona Christi (a Latin phrase, meaning “in the person of Christ”).
In John 20, Christ clearly gives the Apostles the power to forgive sins.
“And when He had said this, He breathed on them and said to them, ‘Receive the Holy Spirit. If you forgive the sins of any, their sins have been forgiven them; if you retain the sins of any, they have been retained.’” – John 20:22-23 (NASB)
From there, the Apostles passed down their “powers” through apostolic succession (a topic I plan on writing about in the future). Those “powers” are possessed by our bishops and priests today.
So, like Devin Rose asked, would you have confessed your sins to an Apostle? If not, you contradict Christ. And, if you would have confessed your sins to an Apostle, it only makes sense that you would confess your sins to a priest.
My first confession was the week before I entered the Church. I was so nervous, but I tried to be prepared: I had done an examination of my conscience and had printed out a version of the standard Act of Contrition. I was ready, or at least, I thought I was. But there was just something unexplainable about that few minutes. I truly felt like I was speaking with God. Like Laban felt with Jacob in Genesis 30:27, I felt that God blessed me, forgave me, and transformed me through His priest.
That is why the Church teaches that Catholics should confess at least once a year (the saintly Cardinal Arinze commented on that here). The Sacrament of Reconciliation is an amazing gift from God, so we should utilize it.
There is nothing wrong with asking the heavenly saints to pray for us.
Many Protestants argue that asking the saints to pray for us is “unbiblical,” while throwing around verses like 1 Timothy 2:5. But they are incorrect.
1 Timothy 2:5 — the infamous “one mediator between God and men” verse — refers to salvation, not prayer. The verse reminds us that it is only because of the graces found through Christ (God Himself) that we are able to have any real relationship with God and reach Heaven. It does not, however, absolutely negate relations with angels or heavenly saints. After all, it was an angel (Gabriel) that spoke to Mary before Christ was conceived in her body, not God Himself.
I was raised in several Protestant denominations. They all placed a major emphasis on Christians praying for each other — which is encouraged in 1 Timothy 2:1-4 and other passages. I would contend that a heavenly saint, one who is holy and in Heaven with God, would have a lot more sway with God than a rebellious sinner on Earth would.
To put that another way, if someone asked you to do something for them, would you not be more likely to help them if they were your best friend, as opposed to a complete stranger? Of course, you may very well be willing to do something for a complete stranger, but you would probably be more willing to do something for your best friend.
And there is evidence in the Bible of the saints praying to God.
“Another angel came and stood at the altar, holding a golden censer; and much incense was given to him, so that he might add it to the prayers of all the saints on the golden altar which was before the throne. And the smoke of the incense, with the prayers of the saints, went up before God out of the angel’s hand.” – Revelation 8:3-4
The word for “saints” in that passage comes from the Greek word hagios. Thayer’s New Testament Greek-English Lexicon says that the best definition of hagios is “most holy thing, a saint”. This would seem to undermine the Protestant assertion that “saints” in this context can only refer to people on earth.
Now, what would the saints be praying for? Themselves? Doubtful. They are in Heaven, so they do not need anything, as eternal life with God is perfect. That really only leaves one option: they are praying for us. And because they are praying for us anyway, how could it be wrong to ask them to pray for us about something specific? It is like interacting with a DJ at an event. He’s playing music anyway, so what is the harm in asking him to play your favorite song?
Here’s my Scripture-based defense of the practice that should answer most Protestant objections:
Matthew 17:3-4 & Luke 9:28-31.
Moses and Elijah (who are clearly heavenly saints, not “saints” in the way Paul would sometimes use the word) are with Christ during the Transfiguration.
The martyrs can talk to God.
From those three passages, we can gather that the saints in Heaven interact with God.
The angels and saints (who, in Luke 20:35-36, Christ says are equal to the angels) are aware of earthly events.
1 Timothy 2:1 & James 5:16.
It is good for Christians to pray for one another.
Now, if the saints interact with God and are aware of earthly events (and can therefore hear us), why wouldn’t they pray for us, considering that it is good for Christians (which the angels and saints definitely are) to pray for one another?
Nothing imperfect will enter into Heaven.
Psalm 66:18 & James 5:16.
God ignores the prayers of the wicked, and the prayers of the righteous are effective.
Because the saints have reached perfection (they are in Heaven), their prayers are more effective than the prayers of those that are less righteous, so that’s why one might ask them to pray instead of asking another Christian on earth or simply doing it themselves.
When I was contemplating converting to Catholicism, one of the biggest factors in my decision was, as silly as this may sound, this clip of the Litany of the Saints being prayed (in Latin!) before the 2005 Conclave:
I was so in awe of the sheer beauty of the procession and the Litany itself. The clip opened my eyes to the glorious splendor that is found only in the Church.
I have asked many saints to pray for me, but my most profound experience was before my conversion, when I — after watching a movie about her life — asked St. Therese to pray for me about something that weighed heavily on my mind. I also asked her to send me a red rose (I was very specific about it being a singular red rose, too) to let me know that she had heard my plea. Three days later (Trinitarian symbolism!), a red rose appeared in my house. It obviously got there due to, at least in part, human action (my grandmother happened to bring it home with her that day), but that does not undermine the importance of it to me. Soon after that moment, my big problem was solved, and it was then that I truly believed in this practice.
Catholics do not “worship statues,” as some Protestants have accused. I know that firsthand.
(All verses are from the NASB translation.)