Why did Christ warn His disciples to beware of the Pharisees? What did they do wrong?  

An entire chapter of the Bible (Matthew 23) is Christ teaching against the Pharisees. 

 The Pharisees were conservative, faithful "church" members. They accepted the Torah as the Word of God. They paid tithes. They believed in miracles (at least in days past). They wouldn't dream of stealing, committing adultery, or such things. You would like living next to a Pharisee as they were model citizens.    But instead of having a living relationship with God, the Pharisees took the Word of God and made just another dead religion out of it.  The Pharisee way of thinking is: keep up appearances. "It doesn't matter so much what you are on the inside, as long as you keep the rules (at least publicly)."  Notice that Christ likened their teaching to leaven, or yeast. Why? Yeast spreads easily and rapidly. Humans are very susceptible to approaching God in this manner, "Keep the rules. Just do these 10 things and you are OK."  Different groups may have different rules, but we all accumulate them. It may be how many times each week you must attend church meetings, or what kind of clothes you must wear, or how you must wear your hair. Or, it may be a list of things you cannot do. But, as long as you meet these requirements, you are in good standing with your group (or denomination).  But, good standing with God is based on only one thing according to the New Testament: faith in what Christ did for us. That alone is what makes us right with God. Not our works, but Christ' work.  1 ... He began to say to His disciples first of all, "Beware of the leaven of the Pharisees, which is hypocrisy.  The Pharisees were hypocrites. That is, they put on an act. What they pretended to be in public was not really what they were like in private. They claimed to be perfect in keeping God's law, but as humans they were all sinners, just like the rest of the people were. Yet, in public, they insisted that others keep the Law perfectly, or be condemned.  They were quick to pass judgment on others, and slow to extend mercy or help. The Pharisees had a legalistic view of God's Word and did not realize that God was merciful and wanted to help people.    who also made us sufficient as ministers of the new covenant, not of the letter but of the Spirit; for the letter kills, but the Spirit gives life.  Using God's Word to "kill" instead of using it to minister life is wrong. God's Word is meant to help people and lift them up, not place such hard demands on them that they cannot ever measure up. When we load people down with heavy loads of rules to keep in order to be right with God, we are ministers of death.    Then Christ said to the crowds and to his disciples,  "The teachers of religious law and the Pharisees are the official interpreters of the law of Moses.  So practice and obey whatever they tell you, but don't follow their example. For they don't practice what they teach.  They crush people with impossible religious demands and never lift a finger to ease the burden.  "Everything they do is for show. . . .  If your "Christianity" is nothing more than a list of "do's and don't's" then you are missing real Christianity, which is a living relationship with Christ.