Of all the criticisms one could lodge against Catholicism, the one that matters most is the simple fact that Catholicism teaches a works based gospel. Scripture is clear: The only requirement to receive God's grace is faith. From Genesis to Revelation, like a banging drum, the Bible tells us our works are filthy rags to God (Isaiah 64:6), because they are why we need salvation in the first place (Romans 3:23; 6:23), but if we will just believe in the name of the Son of God, we can know we have eternal life (1 John 5:13), for He justifies the ungodly (Romans 4:4-5), crediting our faith to us as righteousness (Genesis 15:6).
With this being such an easy case to make, Catholics have been forced to adapt since the Reformation. No longer able to deny what the Bible says, they must instead deny what they say. "We also believe in salvation by faith", they say. "We don't have a works based gospel!" In fact, in my experience, if you say they have a works based gospel without explaining exactly what that means, they will assume you mean they believe you can simply work your way to Heaven.
Now, it's worth noting that although that is certainly not what the Catholic Church has ever taught, either now or throughout history, it does seem the current "head of the Church" does. When a little boy named Emanuele asked if his dead father was in Heaven, the Pope answered "God surely was proud of your father, because it is easier as a believer to baptize your children than to baptize them when you are not a believer. Surely this pleased God very much." He also added "talk to your dad; pray to your dad."
But Papal Infallibility was never supposed to apply to everything the Pope says. He can afford to make even the most damning mistakes when he is not speaking "ex cathedra". Therefore, we can let Francis' error be his own, and never apply it to all Catholics.
What cannot be overlooked, however, are the "infallible" canons of the Council of Trent, several of which anathematise the actual Gospel:
"If any one saith, that by faith alone the impious is justified; in such wise as to mean, that nothing else is required to co-operate in order to the obtaining the grace of Justification and that it is not in any way necessary, that he be prepared and disposed by the movement of his own will; let him be anathema." - Canon 9
"If any one saith, that justifying faith is nothing else but confidence in the divine mercy which remits sins for Christ's sake; or, that this confidence alone is that whereby we are justified; let him be anathema." - Canon 12
"If any one saith, that man is truly absolved from his sins and justified, because that he assuredly believed himself absolved and justified; or, that no one is truly justified but he who believes himself justified; and that, by this faith alone, absolution and justification are effected; let him be anathema." - Canon 14
"If any one saith, that a man, who is born again and justified, is bound of faith to believe that he is assuredly in the number of the predestinate; let him be anathema." - Canon 15
"If any one saith, that he will for certain, of an absolute and infallible certainty, have that great gift of perseverance unto the end,-unless he have learned this by special revelation; let him be anathema." - Canon 16
"If any one saith, that the man who is justified and how perfect soever, is not bound to observe the commandments of God and of the Church, but only to believe; as if indeed the Gospel were a bare and absolute promise of eternal life, without the condition of observing the commandments ; let him be anathema." - Canon 20
Now, I have absolutely no intention of going through the entire Council of Trent and showing its every anti-Gospel canon. Above is just a small selection of publicly available information that I recommend you search elsewhere. Suffice to say for now, whereas the Bible says quite clearly "For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast. For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them." (Ephesians 2:8-10), the Council of Trent says that there are other conditions for salvation than faith. And what are they? You guessed it: Works.
According to the official teachings of the Roman Catholic Church, your works are required for salvation, and if you say otherwise, you are to be considered accursed. But in Galatians 1:8, Paul says that even if the Apostles, or an angel from Heaven, should teach a different Gospel than his, they are to be accursed.
We are therefore faced with the harsh reality that the Catholic Church does preach a gospel of works, and therefore, ironically, are working their way out of God's grace (Galatians 5:4). See, in Scripture, grace and works are separated, not only in terms of being unnecessary partners, but actually being mutually exclusive. Romans 11:6 tells us "...if by grace, then it is no longer of works; otherwise grace is no longer grace. But if it is of works, it is no longer grace; otherwise work is no longer work." Read that again: If by grace, then it is no longer of works, otherwise grace is no longer grace. In Scripture, the reward of our works is counted as a wage (Romans 4:4). Thus, by definition, you cannot be saved by grace and works, any more than you can buy a free product. If it's free, you can't buy it. If you buy it, it wasn't free. In the same way, if you have to work for salvation, it is not by grace. If it's by grace, you cannot work for it.
No matter how much they protest, Catholics cannot deny that they believe in a works based gospel, simply because they believe that on top of their faith, they must work for salvation. They must work to earn it, they must work to keep it, and beyond all imagination, if they work hard enough, they can even contribute to the salvation of others (CCC 1477). Thus, the greatest tragedy of the Catholic Church isn't that they believe a piece of bread is literally Jesus. It isn't that they call the Pope "Holy Father". It isn't that they bow to statues. It's that they will go through their lives constantly fearing that their works are not good enough to assure them of their salvation, and will eventually die and find out... they're right.
You were never going to work your way into Heaven, nor were you ever supposed to. You couldn't. Even believing in God (as the devil himself does), you could give every day of your life to Him, praying, serving, worshiping, you will never be good enough. "But we are all like an unclean thing, And all our righteousnesses are like filthy rags; We all fade as a leaf, And our iniquities, like the wind, Have taken us away." (Isaiah 64:6).
But here's the good news: "For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him." (2 Corinthians 5:21). We don't bring our righteousness to God! We bring His. We are blameless because Christ is blameless, and so He took our blame. We aren't saved by good works, but to them. We receive salvation as a gift (Romans 6:23), not a wage! We could never earn it, but it has been earned. The sole condition? Faith. We are, according to the Bible, saved by faith alone. What else could we add? How could our works contribute to our salvation when our works condemned us in the first place? Therefore, I invite Catholics to receive this glorious free gift, from the hands of God Himself.