In Niagara: A History of the Falls Pierre Berton recounts that Charles Blondin (whom Mary Baker Eddy refers to on page 199 of Science and Health with Key to the Scriptures) crossed the Niagara Falls Gorge on a tightrope many times. On different occasions, partway across he cooked himself breakfast, did a backward somersault, balanced on a chair. He crossed blindfolded, at night, and carried his business manager across piggyback style. He did all that by knowing that he could.
Regarding knowing that you can pray effectively, Jesus said: “Whosoever shall say unto this mountain, Be thou removed, and be thou cast into the sea; and shall not doubt in his heart, but shall believe that those things which he saith shall come to pass; he shall have whatsoever he saith” (Mark 11:23). Knowing that you can bring harmony to every human situation—that you can heal sin and sickness through prayer—is key to following Jesus in this way.
The awesome fact that there is nothing apart from God supports your healing work. Though it may require much spiritual growth to demonstrate this fact fully, we have a right to know that we are knowing that God is all because we reflect His understanding of this metaphysical truth.