This is a question probably impossible to answer in its general form. It is unlikely that all wars in recorded human history have been caused solely by religion, but it is also the case that there are many wars in which religion may have been a contributory factor in the conflict.
To disprove that all wars were caused by religion, one merely needs one counter-example. As religion was not a factor in the Peloponnesian Wars, the claim that all wars have been caused by religion is obviously wrong.
On the other hand, there are numerous examples of wars in which religion was a contributory factor, e.g. the Thirty Years War and the British Civil War. So it is equally obviously the case that religion is a contributory factor.
Even in putatively religious wars such as the Crusades, it is difficult to claim the religion is the only factor involved in the war; many Crusaders found the riches of the east an attraction, and the sack of Constantinople in 1204 consisted of Christians looting the Christian city they were supposed to be saving.
Overall, it is important to examine each case individually and not try to make generalizations which cannot be supported.