John 8:45 And because I tell you the truth, you do not believe Me.
Christ was truth itself, and every word He spoke was truth. But a lot of things that He said were very difficult to believe. I think Jesus Christ would make a statement like that "I tell you the truth" before He was going to say something that was hard for them to comprehend, like these that you posted would be hard things to understand.
Luke 9:27 But I tell you truly, there are some standing here who shall not taste of death until they see the kingdom of God.
John 16:7 But I tell you the truth, it is expedient for you that I go away; for if I do not go away, the Comforter will not come to you. But if I depart, I will send Him to you.
Mat 26:34 Jesus said to him, Truly I say to you that this night, before the cock crows, you shall deny Me three times.
Mark 11:23 For truly I say to you that whoever shall say to this mountain, Be moved and be cast into the sea, and shall not doubt in his heart, but shall believe that what he said shall occur, he shall have whatever he said.
mercy, peace and love
Kat