a) What is the Eighth Commandment of God?
The Eighth Commandment of God is: You shall not bear false witness against your neighbor.
We should always tell the truth, especially in what concerns the good name and honor of others. Hence, we should not lie, judge others as bad without a very good reason, reveal the faults of others without a very good reason, and tell secrets we are bound to keep.
b) Why should an Altar Server know and live the Eighth Commandment?
Honesty is the best policy. The mistakes we commit should not embarrass us, but lead us to humbly learn from them, offer reparation if necessary, and do better next time. With this attitude, there is no need to hide our errors through falsehoods, for even little falsehoods will slowly lead us to much greater ones.
It is easy to see the defects of others, but this should not lead us to speak ill of them. We should rather examine ourselves whether we, too, have these defects, so that we could change ourselves first. Then, we should pray for them, offer sacrifices, and correct them privately in a brotherly way. If necessary, we should also privately inform their parents, teachers, or the parish priest so that they could be properly assisted, especially in serious cases.