We must be careful never to be teaching the false doctrines these verses refer to.
2 Nephi 28:5 And they deny the power of God, the Holy One of Israel; and they say unto the people: Hearken unto us, and hear ye our precept; for behold there is no God today, for the Lord and the Redeemer hath done his work, and he hath given his power unto men;
6 Behold, hearken ye unto my precept; if they shall say there is a miracle wrought by the hand of the Lord, believe it not; for this day he is not a God of miracles; he hath done his work.
Note that "there is no God today" here doesn't mean they literally believed there is no God, rather it is a statement he "hath done his work, and he hath given his power unto men".
Some among us sometimes teach their own version of these false doctrines. Given that the alternative is to admit that they, and most of the rest of us, are still of little faith, they instead say "this day he is not a God of miracles". And that is a rationalization we must not teach.
Don't get me wrong, I don't yet have faith like a grain of mustard seed myself. If I was in the boat when Peter walked on the water, I wouldn't be being congratulated. The Lord wouldn't be telling Peter, "Why don't you have faith like Robertson over there". My faith is nowhere like Peter's yet, who still deserved rebuke for his lack of faith.
And knowing that is terribly important. It is important that we realize that God's willingness to grant miracles hasn't changed since he walked among his disciples. We haven't yet developed the faith. It is not because God has changed his ways. We need that knowledge to get past our own rationalizations. We need someone to teach us to improve our faith. We need to be dissatisfied with the level of our own faith, and to seek improvement.
We believe in the ministering of angels, and even that a man or woman may enjoy learning in the presence of the Father and the Son. We believe that if we had faith as a grain of mustard seed, we would say to a sycamine tree to be rooted up and planted in the sea and it would obey. We believe that if our faith was sufficient, even the very trees would obey us. We believe that rivers would turn out of their course, that the violence of fire would be quenched, that the mouth of lions would be stopped, that we would speak, and prevent the fall of atomic bombs, and the very mountains would obey us.
And it is important we not stop teaching that. There is a world of difference between teaching that our faith is sufficient, but this day God is not a God of (many) miracles, and the effect of teaching, just as Christ kept teaching his own disciples, that it is our extreme lack of faith that prevents us.