For me, one of the greatest things that we can do to reply to Satan's thoughts it to accept that it might not be possible to understand everything. While the road to apostasy is often the slow and gradual one, Satan will also sometimes try to go for broke. Work in one doubt, and then convince us that if that one thing might not be rock solid in our testimony, then the whole Church must be false. Yes, we will always have questions. Yes, some things may not make sense. But God is still there, God still hears us, and we will eventually have all of our questions answered, if we just have a little patience, and alot of faith.
As a side note- When any type of bad thought enters my mind, I've found that the oft-suggested tactic of singing a mental hymn will seriously work wonders. With a little bit of practice, 'The Spirit of God' will now instantly replace any type of temptation, will no effort on my part.
Also, even though it's been said before, I really can't stress reading the scriptures enough. For me, if I ever miss a day, I will often have a problem with temptation the very next day. It's a simple thing that the Lord has asked of us, but by showing the faith to study the scriptures every day, we really prove our devotion to God.
