For the Sadducees say that there is no resurrection, nor an angel, nor a spirit, but the Pharisees acknowledge them all.-Acts 23:8, NASB
The Sadducees and Pharisees were religious leaders who were responsible in teaching the Jewish people the Word of God correctly (Matthew 23:1-3). They were actually pushing people away from God by teaching them to embrace their tradition instead of God’s Word (Mark 7:10-13). Each person they converted through their made-man tradition, they destined them to hell like themselves.
Woe to you, scribes and Pharisees, hypocrites, because you travel around on sea and land to make one proselyte; and when he becomes one, you make him twice as much a son of hell as yourselves.-Matthew 23:15, NASB
There are many Christians who think that they to know the Bible, but when they bring someone to Jesus for salvation, the reality is that they have been brought to a false Jesus who cannot save them.
6 I am amazed that you are so quickly deserting Him who called you by the grace of Christ, for a different gospel, 7 which is not just another account; but there are some who are disturbing you and want to distort the gospel of Christ. 8 But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed! 9 As we have said before, even now I say again: if anyone is preaching to you a gospel contrary to what you received, he is to be accursed!-Galatians 1:6-9, NASB
You need to be careful who you allow to teach you from God’s Word. Everything they teach you, check it out with the Word of God to make sure it is biblical and pray to the Lord to keep you from deception.
A major clue that you are dealing with a false teacher as mentioned in Acts 23:8 is that they don’t believe in what the bible teaches like angels, demons, the spiritual realm, and the resurrection of the dead for those who believe in Jesus Christ. A student of the Scripture believes in everything that the Bible says, but a false one will pick and choose what to believe in to try to mislead. Some on the other hand are sincere in what they believe. The problem is that they were taught a false gospel and now they are spreading it to others.
The biggest and more damaging theology that all false teachers have is the denial of the deity of Christ Jesus. They will bring to you a different gospel that teaches a different Jesus which will give you a different spirit.
3 But I am afraid that, as the serpent deceived Eve by his trickery, your minds will be led astray from sincere and pure devotion to Christ. 4 For if one comes and preaches another Jesus whom we have not preached, or you receive a different spirit which you have not received, or a different gospel which you have not accepted, this you tolerate very well!-2 Corinthians 11:3-4, NASB
Read God’s Word everyday with prayer, asking Jesus for understanding. Also, ask Him to give you boldness to proclaim the good news to a fallen world without fear.