Satan speaking
We know that Satan cannot read our thoughts but can he speak to us through our thoughts or are our sinful thoughts just a product of our sinful nature? In other words, if the Holy Spirit can guide our thoughts, does Satan also have the capability to guide our thoughts or can he only affect our "situations" to manipulate us by creating doubt and other thoughts that hinder our faith? I have heard televangelists and those of that saying things like "those voices in our heads are Satan talking to us" but I find no scriptural basis for that belief other than Old Testament documentation of Satan speaking directly to people like Eve.
You ask important questions. Satan is a formidable enemy (Ephesians 6:12). By his powerful resurrection from the dead, Jesus crushed Satan’s head, just as Scripture had prophesied (Genesis 3:15). Satan has been defeated. “He knows that his time is short” (Revelation 12:12) until he is put out of commission once and for all (Revelation 20:10). Until that time, Satan’s mission is to try to fill hell with as many souls as possible.
While the Bible does describe Satan, a fallen angel, as having powers and abilities greater than ours, it does not give us many specifics. Beyond demon possession, which your questions do not address, we know that Satan “entered into” Judas (John 13:27). The apostle Paul wrote of Satan that “The god of this age has blinded the minds of unbelievers, so that they cannot see the light of the gospel of the glory of Christ, who is the image of God” (2 Corinthians 4:4). Satan certainly knows our weaknesses and can use that knowledge to his advantage in ways we can only imagine.
In your first question you mention our sinful natures. That is an area of our lives that certainly deserves attention. It is so easy for Christians (and the televangelists you mentioned) to blame Satan for all their sins and troubles. I sometimes get the impression that people think that locking up Satan somewhere right now would eliminate their struggles with sin. That kind of thinking fails to realize how thoroughly corrupt the sinful nature is. The sinful nature is an ally of Satan. It is hostile toward God (Romans 8:7). It is the place where evil thoughts originate (Matthew 15:19). It wants nothing to do with God or godliness. It continually wars against our new self (Romans 7:18-25).
The sinful nature is an enemy that needs our continual attention. Recall what our Catechism says: “Baptism means that the old Adam in us should be drowned by daily contrition and repentance, and that all its evil deeds and desires be put to death.” We do that by putting off the sinful nature, rejecting its temptations and confessing our sins when we give in to temptation (Colossians 2:11-12; Romans 6:2, 3, 6, 12, 13). At the same time, we build up the new self through the use of the gospel of Jesus Christ.
Christians long for the day when they no longer fight against the ungodly trio: the devil, the world and our sinful flesh. That day is when our earthly lives come to an end. Until then, we “fight the good fight of the faith” (1 Timothy 6:12).