How should we act toward those who profess to be Christians but
seem indifferent to spiritual things?
Your question reveals you realize just how truly narrow the way is (Matthew 7:13-14). It can be frightening to think that many people in the church (including many whom we love) may not be true Christians.
There are two extremes to avoid in assessing the spiritual status of others.
On the one hand, you must realize that we are not ultimately the judge of anyone's salvation, because you cannot see anyone's heart. Only God can do that (1 Samuel 16:7). And people express their love for God and faith in Christ in different ways.
But on the other hand, you must hold firmly to the truth of God's Word concerning salvation and not offer false assurance to people who may not be truly saved.
If you have a friend or loved one whose lifestyle seriously discredits his or her profession of faith, speak frankly to that person about your concerns. Certainly we have the Lord's own warrant for that in Matthew 18:15-20. If you carefully and lovingly follow the steps Jesus outlined in that passage, you will soon know whether to treat the person as a brother or sister in Christ, or as "a Gentile and a tax-gatherer" (v. 17).
Either way, your attitude toward the person is not to be judgmental, but loving and compassionate.
For further study, consider John's seven-part audio series,
My Brother's Keeper
, on Matthew 18:15-35.