Does it matter what one believes, so long as he is sincere?
Question 1
Answer
But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth:
2 Thessalonians 2:13
NOTE - Doctrine affects the life. Truth leads to life and God; error to death and destruction.
No one would think of saying it matters not what god one worships, so long as he is sincere any more than he would think of saying it matters not what one eats or drinks, so long as he relishes what he eats and drinks; or what road he travels, so long as he thinks he is on the right road.
Sincerity is a virtue; but it is not the test of sound doctrine. God wills that we shall know the truth, and He has made provision whereby we may know what is truth.