In the videoTry this one:
The Catholic Mass 155 AD
Notice that the day of observance is Sunday.
The Eucharist is the real presence of Jesus Christ and not a symbol.
You see that the Catholic Mass in 155 AD is the same Catholic Mass that we have 1,900 yrs later.
Where did Justin learn this from?
Well, we know Paul preached to hold steadfast to your traditions(2thess 2:15)and instituted the Mass(1cor 11:23-25).
Paul is also clear that whomever partakes in the Eucharist unworthily will be guilty of profaning the actual body and blood of Our Lord.(1cor 11:27)
Justin Martyr likely learned from someone who walked personally with the Apostle Paul and was ordained a Bishop by him.