Humanism is man assuming that he is basically the center of the universe and does not need supernatural assistance. Put another, more specific way, it is a secular ideology which espouses reason, ethics, and justice while specifically rejecting supernatural and religious dogma as a basis of morality and decision-making. Many people think they are good just by themselves and do not need a savior, of sorts, to bring them out of anything that could be harming them (the main issue being their sinful condition that keeps them from having God's best for them). Many think they can just do good works to get into heaven, or paradise. Well the truth is that we do indeed need a savior. And we have only One; His name is Jesus (Acts 4:12). He is the only One that could do what we could not do, that is, pay the debt for our sins. Humanism is dangerous because it can cause people to boast about their own good works and then they can become haughty. But the thing is that humanists seem to not understand that the sense of their morality they boast about has to have come from a higher all knowing power. So why not attribute it to God, the true and living God, who expressed the ultimate love in sending His only Son, Jesus Christ, to die for OUR sins on the cross?
Legalism is dangerous too since there are many modern day Pharisees out there that will tell people to do all these religious things, and not even do those things themselves (Matt. 23:4). Legalism is binding, and Christ wants us to be totally free in Him (John 8:36). Yes God has strategically placed leaders in the earth to preach and teach about His Kingdom, but they are not to be legalistic about how they get people to obey the principles of God's Kingdom. We should not go to church, just to go, but to experience more intimate revelation of God's Word and purpose for our lives. We should pray, not to just have something to do, but to really get into the face of God and see what He would have us to do. We should be a blessing to others, not to get something in return, but to give honor and praise to God who is the One who provides us with what we need to bless others in the first place! We should not do what we do for tradition's sake, but we need to do what we do to focus on building up the Kingdom of God.
False doctrines concern things taught that do not line up with the Word of God. God's word is the final authority and we must condemn any false information, and rebuke people who would cause people to err and go the wrong direction. We must be careful to not twist God's Word and use it for selfish applications. In these last days, we have false prophets and false teachers preaching what people want to hear, instead of the truth contained in God's Holy Word. This blasphemy will be dealt with by God in due time, so they'll receive their just reward. We just need to pray as believers that people will hear the real truth, the Gospel of Jesus Christ, turn from the sins, and turn to the living God who is able to save their souls.