Many muslims live in secular law countries where there are other religions and sometimes members of the islamic community convert to other religion. Since Sharia could not be enforced in those countries, which will be the appropiate package of measures to address this problem?
You cannot do anything but to follow the verse:
Invite to the way of your Lord with wisdom and good instruction, and
argue with them in a way that is best. Indeed, your Lord is most
knowing of who has strayed from His way, and He is most knowing of
who is [rightly] guided. 16:125