Not everyone who claims to speak for God actually does.
Matthew 7: 21 "Not everyone who says to me, 'Lord, Lord,' will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven. 22Many will say to me on that day, 'Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?' 23Then I will tell them plainly, 'I never knew you. Away from me, you evildoers!'
The Bible repeatedly warns against false prophets. After Nebuchadnezzar destroyed Jerusalem, Jeremiah wailed,
Lamentations 2:14 The visions of your prophets were false and worthless; they did not expose your sin to ward off your captivity. The oracles they gave you were false and misleading.
Ezekiel railed the spin masters of his day, who covered up for the politicians.
Ezekiel 22: 27 Her officials within her are like wolves tearing their prey; they shed blood and kill people to make unjust gain. 28 Her prophets whitewash these deeds for them by false visions and lying divinations. They say, 'This is what the Sovereign LORD says'_when the LORD has not spoken.
Jesus was very concerned about false prophets:
Mt 7:15 Jesus said to his disciples: "Beware of false prophets, who come to you in sheep's clothing,but underneath are ravenous wolves.
Matthew 24: 4 Jesus answered: "Watch out that no one deceives you. 5For many will come in my name, claiming, 'I am the Christ, and will deceive many. 10At that time many will turn away from the faith and will betray and hate each other, 11and many false prophets will appear and deceive many people.
How do we tell who is a false prophet? Jesus tells us to look at the fruit:
Matthew 7: 16-20 By their fruits you will know them. Do people pick grapes from thorn bushes, or figs from thistles? Just so, every good tree bears good fruit, and a rotten tree bears bad fruit. A good tree cannot bear bad fruit, nor can a rotten tree bear good fruit. Every tree that does not bear good fruit will be cut down and thrown into the fire. So by their fruits you will know them."
Luke 6:26 Woe to you when all men speak well of you, for that is how their fathers treated the false prophets.