The members of those denominations are fallible, they have free will, therefore are capable of allowing Satan's influence. Let me reiterate, the members are fallible, not the Word. Think, for a moment, of church as a physical rehab center. A person who needs to strengthen a knee after surgery will typically go through rehab to gain strength. What if that person, who needs rehab, gets a tech who is weak in knowledge, more damage can be done to the knee than pre-surgery. People attend church to worship God, think of it as spiritual rehab. If a leader succumbs to the temptation of Satan, that leader can do damage to a person that drives them away from Christ. Satan comes as a wolf in sheeps clothing, all words and actions of man need to be tested by fire in order to determine they are truth.