While mentioning an activity related to travelling or going somewhere , we say " Ich gehe in den Park " oder " Ich gehe ins Kino " oder " Ich gehe in die Kirche " . This is an Akkusativ context where masculine noun " Park " or neutral noun " Kino " or feminine noun " Kirche" is preceeded by definitive artikel . Ins = in das .
But if we are already in that place and trying to mention that we are sitting inside or in the place , we say " Ich sitze im Park " oder " Ich sitze im Kino " oder " Ich sitze in der Kirche " . This is a dative context . Im = in dem . Here also nouns of different genders are preceded by definitive pronoun .