Israel had experienced many miracles - the delivery from bondage in Egypt through mighty miracles, the parting of the Red Sea, the manna and water and so on. Even their clothes and shoes did not wear away over forty years. But they did not have the "ears to hear" - the gift of God that we have - so observing all those miracles did not avail them as we can see below and in the prophesy of the Song of Moses.
Yes indeed. Even many who witnessed the miracles of Christ did not believe. God must ready the person to understand. About those who did witness but did not believe I am sure we have all thought "what was the matter with them?". But then we have only to look at ourselves, with all of our advantages, and see when we have not believed or been faithful. We are the same.
The power of God is Jesus Christ, the Living Word of God. When we convey the words of God and our testimony - we have done the very best we can do for anyone.
AMEN! That's all our job is. It is God alone Who saves souls. We never fail nor succeed when it comes to the salvation of another. But it is always such a blessing when God lets us watch Him work. :)