I have seen miracles in other religions. How is it possible when they are not worshiping the only true God?

It is not surprising that there are miracles not only in many other religions, but even among those who do not believe or even acknowledge God. God is the creator and Father of all the peoples of the world and loves each person dearly. He sent His only Son to reveal His love so that whoever knows the Son also knows the Father. (John 14:9) It was this Son who established the Catholic Church, so of course we believe that the fullness of truth is here and there is not a better path that we should take ourselves or suggest to others. 

That being said, God is not limited to the Church and allows all the peoples and religions of the world many good things. In the Nostra Aetate, the Declaration on the Relation of the Church with Non-Christian Religions of the Second Vatican Council, the Church states: “The Catholic Church rejects nothing that is true and holy in these religions. She regards with sincere reverence those ways of conduct and of life, those precepts and teachings which, though differing in many aspects from the ones she holds and sets forth, nonetheless often reflect a ray of that Truth which enlightens all men. Indeed, she proclaims, and ever must proclaim Christ “the way, the truth, and the life” (John 14:6), in whom men may find the fullness of religious life, in whom God has reconciled all things to Himself.” 

Miracles and healings are very needed and important, yet only point to something even more necessary and important: to know who we are, our purpose and that we are loved…in short holiness. For this, of course, we would not want to choose for ourselves nor suggest to others anything except what the Son of God, Jesus, Himself has given us.