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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.