In the evangelical world, “faith” carries strong suggestion of being a crescendo event: faith to move mountains, faith in a healing, faith to step out and obey some calling or decision. It has a sense of screwing up one’s courage, or demonstrating one’s confidence in God at a point of time.
The word “faithful,” however, strongly suggests continuity. To be full of faith and to be faithful have overlapping but distinct meanings at this point. “Faithless,” has elements of both, but hews closer to the one-time, short-term version. I consider faithfulness a very great virtue, and it is clear the writers of the NT thought so, too. We should include more of the current “faithful” meaning in our current usage of “faith.”