A lot of times I've been saying that we have to work for our salvation but I understand that God had already sent Jesus to save us. I know that salvation is free and that we just have to receive it.
Receiving salvation is not like receiving a candy from a friend. We receive it by heart and with all our soul and being. It sounds so easy but it's not. The other side doesn't want us to go to Jesus so they will distract our faith with all the good promises, leaving us with doubts that Jesus can save us.
You ask, "Why would not God stop the opposite side? If He is powerful then He can easily do that."
It's a way of testing our faith. God had given salvation for free but we have to get it our selves. Don't you think it's too much if He would hand it to us and take it for us? Just like the yin and yang, it says we need darkness to know what's light. We need sadness to know and feel the essence of being happy.
Working for our salvation means keeping our faith stronger until it becomes indestructible. We are saved by grace but I believe salvation will be given if we stop doubting.
So how will our faith be tested? There are a lot of ways, like doubting that praying can cure us, help us with our problems, feel bad for our failures, regret the things that we have done, being afraid and the list goes on.
I believe that faith can move mountains. I am working for my salvation through strengthening my faith and for preparing for the greatest challenge that the opposite side wants me to experience just to blame God for my sorrows and let go of Him. I don't want that to happen and that's what I am working for.