Syncretism¶
Is everyone worshipping the same God?
In short, no.
Second temple Judiasm is the soil from which Christianity emerged -- in other words, where Christ was born, died, rose from the dead, and ascended into heaven, ruling creation in the midst of His enemies.
The Logos incarnate
Not modern Judiasm, nor Islam... Modern Judiasm denies that Jesus is the Christ. Islam denies that Christ is God.
1 John 2:23
Whoever denies the Son does not have the Father either; he who acknowledges the Son has the Father also.
This can be a rather difficult topic, as we're forced into the uncomfortable proposition that other beliefs, including many modern Christian beliefs (such as mormonism) are not just wrong but also heretical and blasphemous.
Buddhism
Daoism
The closest we get to syncretism in orthodox Christianity is the revelation that the other "gods" are created beings, thus not "gods" by any reasonable definition. For example, Hinduism is not worshipping the same God, but rather they are worshipping created spirits.
What about heterodox Christians? Do they worship the same God but incorrectly? Or do they worship the antichrist?
To discern, we can know them by their fruits, and where they would lead a believer. Christ meets us where we are -- like a shepherd seeking lost sheep -- and He guides the lost souls back to paradise. The antichrist also meets us where we are, but does not guide as back; in his lies he claims we are good where we are, and that the flock should cone to us, a faux compassion of false acceptance (acceptance of sin and evil as good).
Many modern Christians are participating in the spirit of antichrist. It is the spirit of our times, our modern worldview of continuous destructive revolution.