Why is doctrine so important when salvation is a direct result of believing alone like Abraham?

Why is the doctrine that a Christian subscribes to such as Trinitarianism, Unitarianism, Methodists, Baptists et cetera so important when someone like Abraham was justified on faith alone? God told him to leave and he did and it was accorded him righteousness: *Genesis 15:6* >Abraham believed the Lord, and he counted it to him as righteousness. Paul also reinforced that salvation is by faith alone: *Romans 4:3* >Abraham believed God, and it was credited to him as righteousness.” The two instances above actually seem to teach that believing in Jesus is what actually saves people and not whether or not they were Trinitarian or Unitarian since faith is universal to all Christian denominations. The Jewish saints of the OT who came after Moses did not seek a reason as to why God who is "one" uses the word "us" to refer to Himself; they did not care about doctrine yet they were saved, so why is it so important now?