"...for whoever believes..."
The difficulty of faith at times is that it is too simple. Faith is simply a trust in God. In many ways faith itself is simply the receiving of a gift from God. The difficulty in this is that it is offensive to the human heart.
Of course we want to add something to the gift. Receiving something as valuable as faith (in the end nothing else matters) with no strings attached is simply difficult to take. To desire to add to faith is the natural tendency. But of course since faith is simply receiving a gift the addition of work or merit to it negates the faith.
The great difficulty of Christianity is not that it is too complex, but that its solution to the problem of the darkness in man's heart is too simple. We do wish to climb the mountain and obtain faith by ourselves. But scripture teaches us that faith is simply saying to God, "Lord have mercy on me a sinner."
To describe faith as an act is an exaggeration. Faith is simply an acknowledgement of brokenness and an acceptance of a solution that has already been done by Jesus after he had fulfilled all righteousness on our behalf then he paid all of our sin upon the Cross.
Where is our righteousness? It is in heaven seated at the right had of the Father.