One Gospel: Grace, Not Works

Questions about how we can know who is teaching the same gospel Paul taught, and whether or not Jeremiah 1:5 supports the idea that we pre-existed in a spiritual form.  

• With so many divergent traditions, how can we know who is teaching the same gospel Paul taught? If someone in our age has been taught by revelation, then we should forget traditions.
• Does Jeremiah 1:5 support the idea that we pre-existed in a spiritual form?