Basically, there is always energy lost in any work and is not 100% efficient (such as in the fusion reaction of a star, energy lost in an engine due to friction, etc...)
Therefore the universe is always losing potential energy (you’ll probably know this as entropy). So an infinitely existing Universe would be expected to have 0 energy left in it. Since our Universe doesn’t have 0 energy, it has not always existed.
Interestingly enough, the Bible states that our Universe is subject to decay (entropy).
Energy is not merely “lost”; the operative word ‘lost’ implies a loss of a particular form of energy, into another, less usable form, for the purpose at hand. Energy doesn’t vanish into a vacuum- at most, there is a mass-energy inter-conversion.
Please elaborate on this ‘entropy’ idea of yours, and what the Bible exactly says about the same.