Today I looked up the Dutch word 'belijden' (to confess) in a dictionary. There it said that the word is first attested in the 13th century (1282) and that it is related etymologically to the Lithuanian word 'klykti' (to scream). This is a well-respected dictionary (Van Dale), so I trust that this etymology is reliable.
Now I started wondering how there could be an etymological link between Duch and Lithuanian on such a word dating from the 13th century or before. I'm unsure about the direction, but usually, such indications in the dictionary mean that the Dutch word is derived etymologically from the foreign word.
Is there any significant interaction between the Middle Dutch-speaking low countries and the Lithuanian-speaking baltic around this time that would explain such a link? The word is used nowadays in a predominantly religious setting, yet I suspect the eastern schisma means that there likely was no close religious interaction between both regions? Was there maybe trading interaction between both on the baltic sea and north sea?
Any conjectures from the medievalists among you?
Not a medievalist, but a linguist. There may have been some special connection between the Dutch and the Balts in the 1200's, but this word isn't a consequence of that, but of an older connection between the languages. Dutch and Lithuanian utimately belong to the same language family, Indo-European. The Indo-European languages have a reconstructed common ancestor in proto-Indo-European. Both belijden and klykti seem to descend from the same word in PIE, which is reconstructed as **kel-*.
There are similar words in other IE languages - an Old English word hlīgian 'award, attribute', an Old Frisian word bihlīa (and a modern Frisian word belide) 'confess give a statement', a Serbo-Croat word klicati 'shout', and several others.
This is from M. Philippa, F. Debrabandere, A. Quak, T. Schoonheim en N. van der Sijs (2003-2009) Etymologisch Woordenboek van het Nederlands, 4 delen, Amsterdam which I found at this website.