Sometimes, people receive what they claim to be visions or revelations from the Lord without subjecting such to God's Word. They don't take the time to check that such visions and revelations are consistent with the written Word.
For example, a certain man had a vision. He had been a Christian for many years, but he became sick. While on his sick bed, he had a vision where someone, wearing sandals and a white robe, and covered with a cloud, walked into the room. He started looking from the person's feet upwards to see who it was. As he looked to see his face; the man spoke up and said, "It's not my will to heal you," and disappeared.
Although he couldn't see the face of the one who appeared to him, he believed it was Jesus, because he said he felt the presence of God in the room.
Hence, he accepted that it wasn't the will of God to heal him, and began telling other brethren how it wasn't God's will for him to be healed. But that couldn't be Jesus, or why would anybody think it was Jesus who appeared to the man?
Is it because the person described was in a "white robe," covered with "white cloud?" Or is it because, as the man said, he "felt" the presence of God in the room? None of these is enough for anyone to assume it was Jesus.
You have to understand the interpretation of revelations. If you saw a vision of someone that looked like Jesus, and the person told you something that's inconsistent with the Scripture, then it's questionable; you have every right to reject it.
This is something many people are ignorant of. The will of God is clearly expressed in the Scriptures: God wants you well! In 3 John 1:2, He said, "Beloved, I wish above all things that thou mayest proper and be in health, even as thy soul prospereth."
He'll never contradict His Word. So, if the voice you're hearing contradicts the Word of God, that's a demon talking to you, irrespective of the appearance. Moreover, we're warned in scriptures not to be deceived, because even Satan disguises himself as an angel of light (2 Corinthians 11:14).
Every revelation must be subjected to the Word of God! Don't accept it just because you saw a vision or because you got it from someone whom you think is spiritual; judge it by the Word. A revelation may not necessarily be perfect and accurate, but every revelation must be consistent with the revealed Word of God. This is another reason you must know the Word of God.