Don’t blame yourself for your daughter’s apparent loss of faith; she alone is responsible before God for her decision to reject Him. Yes, you and your church might have done more to prepare her for the challenges she’d face in college, but she must make her own decisions.
At the same time, she isn’t rejecting you—nor should you reject her or spend all your time arguing with her. She is on the edge of adulthood, and as part of this, she’s beginning to test what it means to be independent. In reality, some her age turn away from God—only to come back to Him as the years pass and they grow more aware of life’s realities.
You might ask her how she came to this conclusion. Most likely, she’s been influenced by someone with a strong prejudice against the Christian faith. That person may have come up with arguments she’s never thought of before, and she’s not aware that others have grappled with the same questions—and answered them. (Your local Christian bookstore can suggest helpful resources.) The Bible says, “Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have” (1 Peter 3:15).
Pray for your daughter, that she’ll face honestly the hopelessness and despair that atheism brings in its wake. Urge her also to look more closely at Jesus (and not just those who claimed—perhaps falsely—to be His followers). The Bible says, “No one has ever seen God, but the one and only Son … has made him known” (John 1:18).