Christian Theology: An Introduction by Dr. Alister McGrath
Alister McGrath’s "Christian Theology: An Introduction" is one of the most internationally-acclaimed and popular Christian theology textbooks in use today.
I used this book for my seminary course in systematic theology: https://www.amazon.ca/Christian-Theology-Introduction-Alister-McGrath/dp/1444335146
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I thought that it was a relatively balanced introduction to the wide array of theological customs in the Church.
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https://www.amazon.com/Christian-Theology-Introduction-Alister-McGrath/dp/1444335146
10/10 Would recommend!
Your friend gives a rather weak argument. He makes a claim and makes a huge leap to a conclusion that simply doesn't follow, and which he presumably held even before he developed his argument.
Theology, and philosophy, are interesting subjects that are worthy of study. You can find many better reasoned arguments both for, and against, God's existence in an introductory Theology text. Christian Theology: An Introduction by Alister E. McGrath and An introduction to the philosophy of religion by Brian Davies are two I particularly enjoyed.
It's OK and normal to wonder and doubt. The best advice I can give you is to always remain open to the idea of God, and by extension, to always be open to God's love. Hebrews 11:1 says "faith is the assurance of things hoped for, the conviction of things not seen". You will not find irrefutable proof either way. None of the arguments I've ever read that seek to prove, or disprove, God's existence have been compelling enough to convince someone whose mind is already made up to change their mind. Some are so poor as to be discounted out of hand, and the rest rely on your accepting certain premises.