TO ANSWER THE OP...
The hardest concept for me to grasp is God's omniscience & omnipresence, able to hear millions of prayers at once, & being everywhere at once, from here on earth to the farthest reaches of the universe, at the same time.
The hardest concept for me to image, being the mortal man I am... THAT HE SAVED AN UNDERSERVING SINNER LIKE ME!... Brother Glen
Romans 5:6 For when we were yet without strength, in due time Christ died for the ungodly.
5:7 For scarcely for a righteous man will one die: yet peradventure for a good man some would even dare to die.
5:8 But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us.
5:9 Much more then, being now justified by his blood, we shall be saved from wrath through him.
5:10 For if, when we were enemies, we were reconciled to God by the death of his Son, much more, being reconciled, we shall be saved by his life.
Btw... The Salvation I believe, is 100% Jesus Christ, not what I did for him but what he did for me!