There are people who would say that if God had
really healed or created a miracle that it can not
be lost.

They look at someone who lost their healing and
say that is proof that they were never healed in
the first place.

The truth is that we receive by and and we walk
by faith. Doubts can well cause us to lose what
God has done for us.

Peter's walk on the water was a miracle, but he
took his eyes off of Jesus. By looking at the waves
he began to sink.