That just happened. 

Tim Tebow just hit a home run in his FIRST at-bat ... on the FIRST pitch ... in his FIRST instructional league game. 

Tebow -- who signed a minor league contract with the NY Mets -- bombed it to left center field. 

He was greeted at the plate by his teammates -- who were all going nuts!

FYI -- the pitcher is no joke! John Kilichowski went to Vanderbilt University ... and was an 11th round draft pick of the St. Louis Cardinals in the 2016 MLB draft.