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Scroll down to the Bible text references for "Matthew 16:19" and "Matthew 18:18" on this "biblehub.com" "Strong's" web page "http://biblehub.com/greek/strongs_1510.htm" (which uses "Strong's" to show their translation of the Greek word "estai" as used twice in both Matthew 16:19 and Matthew 18:18), and you will find they translate both sets of those occurrences in both of those Bible texts to "will be bound" and "will be loosed".
Similarly, their "Thayer's" entry here - "http://biblehub.com/thayers/1510.htm" (do a 'find' on "Matthew 16:19" on that page) for that Greek word translates it as "shall remain bound" and "shall remain loosed".
Here are the "biblehub.com" interlinear entries for those two texts:
eimi - Definition: I am, exist.
1510 eimí (the basic Greek verb which expresses being, i.e. "to be") am, is.
Note there is no beginning or end. The apostles did not initiate. It already existed. It had been known forever in heaven. As I said, the apostles simply ddclare d what already existed.