Prayer, prayer, and more prayer! Although I’ve had low periods in my faith life, it was a lot more helpful when my mom stopped pushing me and trying to guilt me into going to church (this was even as an adult in my 20’s living far away from home). I am now quite deep in my faith and involved in my Parish (I even moved back to my hometown, but I was single so had the luxury of trying to start a little life here). But this was due to my own research, my own decision, and because I had seriously been thinking about my relationship with Christ.
My brother married an atheist and they have a child together (who is not baptized). It is unusual because we had quite a strong Catholic upbringing, but he does not attend Church and their wedding was done by a justice of the peace in their backyard. However, his wife tells me he is still quite aware of God and the Catholic Church. Both my mom and I pray for him (and their little family), and I occasionally will send a book he might like. Both he and his wife were open to putting up little things around their house, like those wall decals with a Bible verse.
Prayers and space to find their way are very helpful, as it worked for me!