Paul taught the people to withdraw themselves from anyone who teaches a doctrine other than the apostles' doctrine.

2 Thessalonians 3:6 "But now when Timotheus came from you to us, and brought us good tidings of your faith and charity, and that ye have good remembrance of us always, desiring greatly to see us, as we also to see you:"

Let God be true and every man a liar.

Apostles are given the responsibility of protecting the entire flock of God from the attack of the enemy, but with so many voices in the world, each one claiming to be the one that should be listened to, how can you know?  You are responsible to be close enough to God so that you can tell the difference between false apostles and real apostles.

Read the complete definition of "Apostle."