In terms of becoming a christian what part does doctrine play?
Jesus asked His disciples, "Who do you say that I am?" Our answer to that question matters, because what we say about Jesus matters. It matters because what we believe shapes not only how we see God, how we see Jesus, see Jesus' message and mission, but also how we understand our place in the world and our engagement with other people and with all of creation.
If I believe that this material world doesn't matter, then how I treat the world and those things living in it will be different than if I believe that this material world does matter.
If I believe that God loves everyone, and I am supposed to imitate God, then I will understand that I am supposed to love everyone as well. If, on the other hand, I believe that God only loves some people based on how "good" they are, then that's going to show up in how I treat other people as well.
All of this stuff matters. Because belief and action are intrinsically connected. What is in our mind, what is in our heart, and how we live are all interconnected.
This is ulatimately what St. James is getting at in his epistle, that faith without works is dead. James is not contradicting what Paul was saying about our justification before God. James, instead, is talking about how what we believe is supposed to be expressed in how we live our lives. With the tongue we bless God our Father and with that same tongue we curse our fellow man made in God's image, this should not be so. This is why James can say "even the devils believe, and they tremble". Or look at what St. John says in his epistle, that love is of God because God is love, and therefore we show that we know God by our own love; the one who does not love does not know God, etc--John isn't saying we will never fail and sin, because he also says, "If we say we have no sin then the truth is not in us, but if we confess our sins God is faithful and just to forgive us our sins". But if we believe in God, then the God of all love, with that love with which He loved us, should be a catalyst that pushes us toward love.
What we believe matters. It's all over Scripture. It's why St. Paul wrote to hold firm to the traditions/teachings which were passed on from him and the rest of the apostles. It's why St. Peter writes to give answer to the hope that is in us. St. Jude writes to earnestly contend for the faith and not give in to the destructive opinions of heretics. Over and over and over again, if anyone preaches another gospel it is anathema. There is one faith, one hope, one baptism, one Church, one Lord Jesus, one Holy Spirit, one God the Father. The one who preaches, such as St. Timothy, is to preach the word in and out of season and do the work of an evangelist.
Again, over and over. This all matters.
Not because we are "saved" by doctrine. But because what is taught, what is confessed, and what is believed matters--it matters because God matters, because Jesus matters, because Jesus' cross matters, because Jesus' teaching matters, because His resurrection matters, this Gospel matters, the Holy Spirit matters, this world matters, our neighbors matter, the Church matters. It all matters.
-CryptoLutheran