I haven’t heard the first one regarding Christ (it is common to refer to penitents as those who knock upon the door of mercy). The later is a common image, the one who opened the door of mercy, the one whose door is open to those who knock, the one who opened the door by His death, etc.
There are actually a couple Syro Malabar liturgical hymns that state “Vadil thuranu pidichavanam” which literally translates to “The one who held the door open”. However like MorEphrem stated, I haven’t really heard Christ being referred to as “The one who knocks” at least in my Church. However I have heard some Latin Catholic hymns such as “Somebodies Knockin at Your Door” which is an entire hymn that states Christ is knocking at your door.
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