Before discussing further about Bitcoin halal or not, religion has its own way to forbid and justify investment money to its people:
Usury is one of the prohibitions written in the Bible, Muslims, Christianity, Judaism, Hinduism and Buddhism all religions prohibit usury.
Example:Muslims are forbidden to take usury and involve themselves with usury. Its clear prohibition comes from several suras in the Qur'an and the Hadith of the Prophet Muhammad. Allah permits buying and selling and forbids usury, and every business activity must be based on sharia principles and prudence.
Leviticus 25:36-37
"You shall not take interest or usury from it, but you must fear your God, so that your brother may live among you. Do not give your money to him by asking for interest, nor do you give your food by asking for usury."
Jews were forbidden to practice taking flowers.
For that, let's review about Bitcoin whether Bitcoin is included in the Usury category or in the buy/sell category.Usury: borrowing is signaled by excessive interest, as in the quote above, all religions are prohibited, not halal.
Bitcoin: Bitcoin is currently only traded as in general selling and buying 'investment' means you buy and hold for a certain period of time, if the price goes up you sell, if the price goes down you sell it means you lose.
Many Muslims do 'invest' in gold, buy when it's cheap and keep it and sell it when the price is high, why not say it's not forbidden, why is it halal, in the case of gold investment, many people do it, especially Muslims.
Conclusion: in Bitcoin elements that are happening today are: buying/selling, meaning: halal.