He (kind of) had to add the cosmological constant to GR just so the universe wouldn't collapse onto itself and instead be static. The idea of the unchanging universe was maybe partly due to personal or religious preconceptions. So at least in his mind the universe was static, which of course was proven "wrong" by the discovery that the universe is expanding. This was not a proper scientific theory of his, rather a preconception, like his aversion to the randomness of Quantum Physics (again, religion it appears).

Of course, being Einstein, he was again on the right side of the argument when the universe was much later discovered to be accelerating, again requiring a cosmological constant (or some similar fudge factor).