In the 7th and 8th centuries, Muslims conquered the Levant and North Africa. All the witnessing did not stop the conversion or replacement of Christians by Muslims. The Spanish reconruista was not done by witnessing, but by the sword. The only place where witnessing works is in Southern Africa, where Muslims do not have fukll legal control and where Islam is new.
If you want to witness, witness to the atheist Europeans selling out their civilization.