(This is differently emphasized version of the basic argument to be found at http://charltonteaching.blogspot.co.uk/2014/07/christians-dont-really-believe-that-god.html )
The major constraint on the omnipotence of the Christian God is Christ: specifically the incarnation, death, resurrection and ascension of Jesus Christ.
For Christians, the situation is that before the incarnation of Christ, God could not accomplish the salvation of Man; and after the incarnation of Christ, he could.
Therefore, in Time - the Christian God is not omnipotent.
This is such a very large and very significant constraint on the omnipotence of God, that to non-Christian monotheists it destroys mono-theism, in a way which they find shockingly offensive to God.
The only sense in which the Christian God could be legitimately described as omnipotent, is by having God located outside of Time - in the sense that everything that will happen already has happened.
And therefore, for God to be omnipotent requires that the incarnation of Christ had already happened before it actually happened.
If, and only if, a person can make sense of Life and Salvation on that basis (the ultimate-simultaneity of all things past, present and future - and the ultimately-illusory nature of Time, change, progress etc.), then you will be able to make sense of the Christian God being described as 'omnipotent'.
Otherwise, it is necessary to be clear that the Christian conception of God is not omnipotent - and to try and deny this is to provoke confusion.