All things are possible with God (Mk. 10:27)
In order to have faith in God for anything, we must be certain of the realm of possibility. This realm is boundless.
There are two concepts to be examined here: certainty and possibility.
One must live with both in mind in order to have faith.
In fact, the sphere of certainty lies within that of possibility.
One can say that he is certain that his prayers will be answered, but if he is not willing to allow his mind the possibility that it will not be answered, his faith will be hindered by doubt and will fail.
This is akin to the mental conundrum: "Try not to think of a pink elephant."
The more you try, the more the elephant is transfixed within your mind.
Only by allowing the thought to move through your mind like a weed through a river will it cease.
Faith in God for anything, including salvation, requires a mind free from fear of doubt and even fear itself. But how to be free from fear?
"Perfect love casts out all fear." (1 Jn. 4:18)
"Be perfect, as your Father in heaven is perfect." (Mt. 5:48, 19:21)
"This is love for God: that we obey his commandments." (1 Jn. 5:2-4)
"Fear the Lord and obey his commands, for this is the whole duty of man." (Eccl. 12:13)
"In everything, do unto others as you would have them do unto you, for this in turn sums up the Law." (Mt. 7:12)
"I can do all things through Him who gives me strength." (Phil. 4:13)
"The reason you do not receive is because you do not ask." (James 4:2)
When we are free from fear we will not hesitate to love our enemies by listening to them and doing our best to understand them, and thus being all things to all men, in the hope that some might be saved.