Forced conversions and a tax rate of 100% on everyone would be unjust. However, a four-fold penalty on thieves, assessed on the value of property they've stolen, is Biblical. Additionally, I see nothing unjust about progressive income tax, where the wealthy would be required to pay 90% tax on all income over $10,000,000 per year.
Oh, fair is fair in communism--everybody should pay at least 90% in communism. Young people should pay even more.