[They will be told], "That is because, when Allah was called upon alone, you disbelieved; but if others were associated with Him, you believed. So the judgement is with Allah, the Most High, the Grand."
That is because, when God was called to alone, you disbelieved; but if others are associated with Him, then you believe. Judgment belongs to God, the All-high, the All-great.
(The answer will be:) "This is because, when Allah was invoked as the Only (object of worship), ye did reject Faith, but when partners were joined to Him, ye believed! the Command is with Allah, Most High, Most Great!"