The Church teaches that the normal means of indwelling of God in our souls begins in Baptism. Notice the emphasis on normal. We don’t try to limit God by saying this is the only means of the beginning of the indwelling.
The Catechism teaches this:
[LIST]Baptism gives us the grace of new birth in God the Father, through his Son, in the Holy Spirit. For those who bear God’s Spirit are led to the Word, that is, to the Son, and the Son presents them to the Father, and the Father confers incorruptibility on them. And it is impossible to see God’s Son without the Spirit, and no one can approach the Father without the Son, for the knowledge of the Father is the Son, and the knowledge of God’s Son is obtained through the Holy Spirit