what is the purpose of...
existence?
the purpose of existence WAS to coexist with the Father and live in eternal communion with Him... until the first sin. now the purpose of existence is to be reconciled with Him because of Jesus Christ, and live our lives with Him as our guide. existence outside of God's covenant is vain and without His covering. the purpose of existence is to live as well as we can with God in our lives. choosing a life without Him---is choosing eternity without Him.
being a christian?
the purpose of being a Christian is to be able to have a relationship with God again, and to spread truth and love of Jesus to as many as who will hear so that they can be reconciled with God once again as well. when you're a believing Christian, not only are you one with the Father again, but you're promised a painless eternity.
believing in God?
you will believe in something no matter what. yourself. books. people. you WILL look to something for guidance. your parents, your friends, your addictions, etc. to believe in God is to recognize that the reason everything is even here is because of some kind of conscious being who could create other conscious beings, and put so much detail into the universe's design that it couldn't possibly all be just one big accident. something can't come from nothing. nothing comes from nothing. the purpose in believing in God is choosing the God who chose you first.
do i have a purpose?
Let me ease your anxiety by letting you know the main purpose God intends for you is to simply abide in relationship with Him. accept Jesus into your life, and have a relationship with the Father. let Him guide you through life. that is the first and main purpose you're given. However, God is the ultimate master of creation and gives many gifts and passions to humans--from the Holy Spirit and from His will on your life. Most likely, whatever you love/are gifted in, the Lord wants to use that gift He imparted to you for His glory so that more people can be reconciled with Him. sometimes He gives passions just to give them, and uses you in other ways. for example, I absolutely loved playing basketball--however God didn't create me to go to the WNBA for His glory. He did give me that passion though, and rather uses the spiritual gifts He has placed in me. sometimes your fears are part of your purpose. i absolutely despise public speaking, but I'm almost 100% positive it will be apart of His plan for me despite me being reserved. the best thing to do is to ask God how He wants to use you, and what career path you should go into. and don't worry about Him answering, He has a way of letting you know no matter what.