—Deuteronomy 13:1-3 (NASB)
False prophets can prophesy something which actually comes true. They can even perform an actual sign or wonder. Their actions can perfectly imitate those of true, godly prophets. But the true test of ALL signs and wonders as to whether or not they are truly come from God is the accompanying message. The explanations and messages accompanying the signs and wonders of the true prophets of God magnify the One True God; those of the false prophet entice observers to follow another, very often the false prophet himself.
There is no biblical loophole to allow someone to act contrarily to their beliefs. No matter how magnificent the visible display, it is negated or substantiated by the accompanying message to reveal whether the prophet follows the One True God or another. The lesson of all God's signs and wonders as documented throughout the Bible—whether they occurred historically or are yet to come—is that they are accompanied by a clear message. For His followers the message drives them even closer to Him, whereas for unbelievers it amounts to an indictment of their unbelief. God never provides signs and wonders without an explanation that reinforces His divinity in some way. Therefore we can identify false activities clearly by their accompanying message. Either way, no sign means anything without an explanation of what witnesses are to do as a result of it, otherwise it will have little if any real impact.
At one time or another, we have all probably asked God for a sign. But we have something that is greater than a sign that is not open to interpretation: God's Word. His Word is the necessary proof of the legitimate origin of signs and wonders, but signs and wonders are not necessary to justify His Word. It's already in our possession and can provide every answer we'll ever need. In fact, we might need to consider the strong probability that when we actively seek visible signs and wonders that we're actually undermining the very Word of God necessary to clarify them. In the final analysis, all we need is His Word, and He's already given it.†††