Nothing is known or can be known about the nature of God. Isn’t it a bit of a stretch that Pastor speak like God exist?
No. Regaridng “nothing is known”, there are lots of things in the universe that man doesn’t know about yet, and lots of things in the past that man didn’t know about for centuries (microscopic creatures for example) but it didn’t make those things any less real just because Man didn’t know about them. Regarding “nothing can be known about the nature of God”, that’s debatable, since God’s son, who is also God himself in the second person of the Holy Trinity, appeared in human form on earth and was personally known to people and his existence historically recorded as well as recorded in Scripture; also many believers have some personal experience of God’s love or God guiding them in some way.
In his opinions, preachers are probably scammers
Some preachers are scammers and some are not. If your friend wants to make broad generalizations about groups of people, rather than making a personal decision about each of their sincerity, that’s his problem.
based on a book written by man, which we called the bible.
The Bible may have been physically written by humans, but it was written under divine inspiration and is the Word of God. It is therefore not just some “book written by man”.
If there is no promise of a afterlife, would Christian still be bothered believe in God?
The entire point of God is that life and love are eternal, while our life on earth is just a short time period of our existence. You cannot separate the two things, God and afterlife. Or if you did, then we wouldn’t be discussing God anymore, as in the real God. A God with no afterlife would be some fictional creature made up by people like your friend.
Since Christian are doing good things for earning brownie points to Heaven.
That’s not why a true Christian “does good things”. We “do good things” because it’s how we’re supposed to act towards others. No amount of “good things” we could do would be sufficient for us to merit Heaven. God decides who goes to Heaven and it’s not based on the person doing 145522121 good things, it’s based on the love in their heart for God and neighbor.
Does it make them doing good less authentic?
No, because anybody who was only doing good things for “brownie points” towards Heaven would be missing the point and God would probably be looking rather askance at that.
As I said, we do things out of love, not to get something for ourselves.
Doesn’t that make unconditional love a fallacy?
No, God doesn’t love somebody extra for doing good things. He loves everybody the same.
He may be pleased with the works of person A more than he is pleased with the works of person B, but that doesn’t mean he loves person A more than person B. It’s like a mom with two kids, one is good and one is bad, but a normal mom loves them both, she doesn’ t just love the good kid.
I hope this helps