Adam Clarke on Esther 8:17

Many - became Jews; for - fear - These were a species of converts not likely to bring much honor to true religion: but the sacred historian states the simple fact. They did profess Judaism for fear of the Jews, whether they continued steady in that faith or not.

It is only the Gospel which will not admit of coercion for the propagation and establishment of its doctrines. It is a spiritual system, and can be propagated only by spiritual influence. As it proclaims holiness of heart and life, which nothing but the Spirit of God can produce, so it is the Spirit of God alone that can persuade the understanding and change the heart. If the kingdom of Christ were of this world, then would his servants fight. But it is not from hence.