Many find great strength in this passage from Mark 11:
"For verily I say unto you, That 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)
In order to understand having the faith of God, and not doubting in one's heart, consider what Jesus has made us to be:
"Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new." (2 Corinthians 5: 17)
No matter what a man says, if He is not a new creature, a new creation in Christ, then whatever he says can have no power.
Why? We can only come before God the Father in His Name:
"These things have I spoken unto you in proverbs: but the time cometh, when I shall no more speak unto you in proverbs, but I shall shew you plainly of the Father. 26At that day ye shall ask in my name: and I say not unto you, that I will pray the Father for you: 27For the Father himself loveth you, because ye have loved me, and have believed that I came out from God. 28I came forth from the Father, and am come into the world: again, I leave the world, and go to the Father." (John 16: 25-28)
We pray in His name, because we have His authority, and we have His authority because we have been made a new creation in Christ Jesus, sons of God:
"1Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not. 2Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is. 3And every man that hath this hope in him purifieth himself, even as he is pure." (1 John 3: 1-3)
"Herein is our love made perfect, that we may have boldness in the day of judgment: because as he is, so are we in this world." (1 John 4: 17)
We can rest assued that we will have what we say, because we believe on Him, Jesus Christ, and we do not doubt that He has done a perfect work at the Cross:
"5If any of you lack wisdom, let him ask of God, that giveth to all men liberally, and upbraideth not; and it shall be given him. 6But let him ask in faith, nothing wavering. For he that wavereth is like a wave of the sea driven with the wind and tossed. 7For let not that man think that he shall receive any thing of the Lord. 8A double minded man is unstable in all his ways." (James 1: 5-8)
Why are so many Christians double-minded? They do not believe that Jesus has put away all our sins forever, but rather that something more must be done, that we must do.
When we believe and do not doubt in our hearts, then indeed we will have what we say.
This issue of not doubting in our hearts, of having the faith of God, is all about becoming a new creation in Christ:
"20I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me. 21I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain." (Galatians 2: 20-21)
One of the key verses for Hagin was not just Mark 11: 23, but 2 Corinthians 5: 17, for what is the point of speaking over and over, if you doubt in your heart that you are a new creation, and that God hears you:
"And if we know that he hear us, whatsoever we ask, we know that we have the petitions that we desired of him." (1 John 5: 15)
We are a new creature, a new creation in Christ, and as His Word dwells richly in us, lo and behold -- you will have what you say!