Hebrews 11:1 says, faith is the substance of things hoped for, the evidence of things not seen.
Abel just believed in God. He was considered as a man of faith.
Even though Abel died as a martyr, his life still speaks today of his faith - knowing God, trusting in Him and moving along with God. Abel believed it was God who created him, everything in the past was God and the hope for the future is God.