I understand what you’re going through. I have a 19 yo son who has also stopped practicing the faith. There are some very good suggestions from other posters about prayer, being the best and most humble Catholic you can be, being a living witness to what your faith has meant to you.
I would also add these thoughts: fast for the intention of your child’s conversion. Make use of sacramentals like placing a green scapular in some discreet location in the child’s room. Pray a rosary every day for this intention. Ask St. Monica to pray with you and to intercede for your child.
I like Bayern’s idea about an indepth conversation to find out what underlies this issue. However, I would pick the time carefully, and you may have to abandon the idea if it starts to be an argument. I strongly believe that young adults are not won over to the faith by strong arm tactics (forcing to attend Mass as a condition of living under your roof), or by contentious relationships.
You may find, as I’ve found with my own child, that they are avoiding the Church as a way to avoid dealing with some sin or sinful activity in their life. One way to “deal” with this (in their opinion) is to just deny that the Church is right about the issue being sinful, so they just won’t agree with the Church. If this is the case, I’ve found it’s helpful to find ways of discussing God’s mercy, and how thankful you are for the sacrament of reconciliation in your own life. How you have been healed by the encounter with Christ in confession ot a priest.
Don’t give up. St. Monica prayed for many years before her son converted. As for my case, I can see some cracks in my son’s armor. Just last week, he was telling me about his indignation at a college history professor who was misrepresenting the teachings of the Catholic Church and the events of the reformation. This past Christmas, I saw him wrestling with whether or not to attend Mass with us. He balked at the last minute, but the next day asked me lots of questions about who was there, was it crowded, what did Monsignor say in the homily. So, there are hopeful signs.
The best advice I can give you is to never give up. Pray, live a Catholic life. And talk to your child less about God, talk to God more about your child. I pray you will have the peace of knowing that God wants your child back even more than you want the child to come back.
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