Answer: The most principle to note is that we cannot unilaterally change another even if the person is our own wife. We cannot force them to convert and become a true Christian. The Bible only commands us to change ourselves. In fact, trying to change others can result in the loss of a friend, a divorce, or even worse.
God reserves the right to change any human, including your wife, and make them one of his spiritual children. Jesus reveals this authority when he stated, "No one can come to Me unless the Father, Who sent Me, draws him; and I will raise him up at the last day." (John 6:44).
It is understandable that you would be concerned regarding the eternal destiny of your wife. It is comforting to know, however, that people like your mate, even if they do not change and become true Christians in this life, are not bound for an ever-burning Hell at death (see our articles on the subjects of Hell, Purgatory, and God's "second chance" at salvation).
Our heavenly Father has a plan for every human. His plan for your wife might be to open her mind and convert her later.
What to do
Do not flaunt your piety to your mate or to others. Do your bible studies in private and do not push your favorite scriptures onto her. Ask God, if it is His will, to call your spouse in this life. If your wife has a question, answer it as simply as possible. This will leave the way open for more queries.
Set a simple but Godly example in everything you do before your wife. If you have little children, read them a story from an illustrated children's bible at bedtime and teach them to pray. Be patient in traffic, show kindness toward everyone, and be honest in all your dealings. In this last regard, a friend told me this experience from her bus-driver father's life.
A well-known minister was coming to town, and pictures of him were everywhere: on billboards, in newspapers, in store windows.
The minister boarded the bus to the town where he was to preach. As he counted his change, he discovered the driver had given him too much. "Oh, well," he mused. "It's only a quarter. The driver won't miss it." However, his sense of honesty told him it was not his, so he should return it. The mental argument went on and on but, when he arrived at his stop, the minister told the driver he had given him too much change, and returned the money.
"I know," said the driver. "With all the pictures everywhere, it was easy to recognize you. I gave you the extra quarter to see if you really practice what you preach." When he got off the bus, the minister slumped down on a bench and cried, "Oh God. I almost sold you for a quarter."
It is up to your wife to decide if she feels God is calling her and he wants her to repent, convert, and live the life of an obedient Christian. Your behavior, however, is best guided by what the Apostle Paul wrote to the Corinthians.
If any brother has an unbelieving wife, and she consents to dwell with him, let him not divorce her . . . the unbelieving wife is sanctified in the husband; otherwise, your children would be unclean, but now they are holy (1Corinthians 7:12 - 13).
Do whatever you can to live peaceably with your wife (Romans 12:18) and leave it to our Father in heaven to convert her.