It's not necessarily a case of duplicity, teaching general beliefs before going on to more 'difficult' ones. The Bible uses the analogy of new Christians feeding on "the milk of the word" before being able to progress to "the meat of the word". That's in 1 Corinthians 3:2 - "Brothers, I could not address you as spiritual but as worldly - mere infants in Christ. I gave you milk, not solid food, for you were not yet ready for it. Indeed, you are still not ready. You are still worldly."
But you can see that that is not the same as deliberately with-holding deep teachings that would never be accepted without a progression of easy stuff first! This speaks of worldly Christians just being immature, spiritually.
The point about all groups like the Mormons, JWs, Scientologists, Masons, Gnostics, Kabbalah etc is that they are not laying the foundation God requires before 'building' on it. In 1 Corinthians 3:10 Paul continues,
"By the grace God has given me, I laid a foundation as an expert builder and someone else is building on it. But each one should be careful how he builds. For no-one can lay any foundation other than the one already laid, which is Jesus Christ."
Only if the good news about what Jesus did for sinners is established first (the milk of the word) can progress be made. Jesus likened himself to a foundation of rock which, if built upon, will ensure the building remains intact when storms rage against it. Alas, groups often build on a foundation of sand because they have not built the denomination on the reality of just who Jesus is. The Jesus of the Mormons and the JWs is not the Jesus of the Bible, because they have a created Jesus, a creature, a demi-god.
Here is the biblical explanation; "Though by this time you ought to be teachers, you need someone to teach you the elementary truths of God's word all over again. You need milk, not solid food! Anyone who lives on milk, being still an infant, is not acquainted with the teaching about righteousness. But solid food is for the mature, who by constant use have trained themselves to distinguish good from evil.
"Therefore let us leave the elementary teachings about Christ and go on to maturity, not laying again the foundation of repentance from acts that lead to death, and of faith in God, instruction about baptisms, the laying on of hands, the resurrection of the dead, and eternal judgment." (Hebrews 5:11 - 6:3)