Dedicated to the Lord Jesus Christ Who loved us, gave Himself for us, and washed us from our sins in His own Blood; and to the Church, which is His body.

Mark 11:23

"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."
Mark 11:23

Assuredly means 'amen' which is a powerful expression of faith.

Jesus using this word, was making a confident declaration about making confident  declarations.