And he said to him, Truly, truly, I say to you, Hereafter you shall see heaven open…
is simply the familiar "amen!" which properly is an adjective meaning firm or steadfast, and is used in two connections. Sometimes it precedes an assertion which it confirms, in which case it may be paraphrased by "Thus it certainly is." Sometimes it follows a prayer which it sums up and reiterates, and in that case it may be paraphrased by "So may it be." Doubled it has the force of a superlative, "Most assuredly." It is heard only from the lips of Christ. It becomes no other lips.
(A. Maclaren, D. D.)
Parallel VersesKJV: And he saith unto him, Verily, verily, I say unto you, Hereafter ye shall see heaven open, and the angels of God ascending and descending upon the Son of man.