"I tell you the truth." This phrase adds emphasis to what Jesus says next.
# the tax collectors and the prostitutes will enter the kingdom of God before you do
Here "kingdom of God" refers to God's rule as king. AT: "when God establishes his rule on earth, he will agree to bless the tax collectors and prostitutes by ruling over them before he agrees to do that for you" (See: [[rc://en/ta/man/translate/figs-metonymy]])
# before you do
Possible meanings are 1) God will accept the tax collectors and prostitutes sooner than he will accept the Jewish religious leaders, or 2) God will accept the tax collectors and prostitutes instead of the Jewish religious leaders.