We see the connection again as Jesus addresses Peter and the other disciples in Mark 11:22–23. “So Jesus answered and said to them, ‘Have faith in God. For assuredly, I say to you, whoever says to this mountain, “Be removed and be cast into the sea,” and does not doubt in his heart, but believes that those things he says will be done, he will have whatever he says’ ” (emphasis added). In all these Scripture portions, you can see that believing and having faith are the same exact thing.
In English, the opposite of belief is unbelief. The prefix “un” makes it the opposite. But to make the opposite in Greek, the prefix “a” is added, turning pisitis (faith) into apisitis (unfaith). In English we don’t have unfaith; we simply just call it unbelief. And apisitis (unfaith) translates into English as unbelief. Also connected with this four-letter stem, P-I-S-T, we have the adjective pistos, which means faithful or believing. From the opposite of this, using the prefix “a,” we have apistos, meaning unfaithful or unbelieving. All these different words with the same P-I-S-T stem occur hundreds of times in the New Testament.
If you examine these verses, you will find a massive, powerful theme ordained by God: Faith is the key to experiencing all that God has promised. This strong and pervasive theme saturates the entire teachings of the New Testament Scripture. In English, we do not see it clearly because it is fractured into two separate parts. But God has given us faith, or believing Him, as the single most necessary thing for living a life of all that He has promised. That one Greek word from the stem P-I-S-T is central to the revelation of Jesus Christ in the New Testament as Savior, Healer and Provider.