The only thing they have in common is the prefix ἀπο-, which means "from." The verbs ἀποκαλὐπτω and ἀποστέλλω, from which ἀποκάλυψις and ἀπόστολος derive, have nothing semantically to do with one another, nor does either verb imply "knowledge" in classical literature. Herodotus uses ἀποκαλὐπτω to mean "uncover" (i.e. a person's head, 1.119), while Plato uses it for uncovering a person's chest (Protagoras 352a). Again, only ἀποστέλλω (not ἀποκαλὐπτω) occurs in Sophocles, where it signifies sending a person away (Electra 71). (Cf. also Euripides with this meaning in Medea 281.)
The noun ἀποκάλυψις is not attested in classical literature until Plutarch, in whom it signifies an "uncovering" of hidden springs (Life of Aemilius Paullus 14.3.1).
Ἀπόστολος, on the other hand, occurs frequently in classical literature, where its usual meaning is "emissary" or "ambassador."