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.

15 “Moreover oif your brother sins against you, go and tell him his fault between you and him alone. If he hears you, pyou have gained your brother. 16 But if he will not hear, take with you one or two more, that q‘by the mouth of two or three witnesses every word may be established.’ 17 And if he refuses to hear them, tell it to the church. But if he refuses even to hear the church, let him be to you like a rheathen and a tax collector.
18 “Assuredly, I say to you, swhatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.
19 t“Again 4I say to you that if two of you agree on earth concerning anything that they ask, uit will be done for them by My Father in heaven. 20 For where two or three are gathered vtogether in My name, I am there in the midst of them.”
o Lev. 19:17; [Luke 17:3, 4; Gal. 6:1]; 2 Thess. 3:15; [James 5:19]
p [James 5:20]; 1 Pet. 3:1
q Deut. 17:6; 19:15; John 8:17; 2 Cor. 13:1; 1 Tim. 5:19; Heb. 10:28
r Rom. 16:17; 1 Cor. 5:9; [2 Thess. 3:6, 14; 2 John 10]
s Matt. 16:19; [John 20:22, 23; 1 Cor. 5:4]
t [1 Cor. 1:10]
4 NU, M Again, assuredly, I say
u [1 John 3:22; 5:14]
v Acts 20:7; 1 Cor. 14:26
The New King James Version. Nashville : Thomas Nelson, 1982, S. Mt 18:15-20
 

18:15 The prescription for church discipline in vv. 15–17 must be read in light of the parable of the lost sheep in vv. 12–14. The goal of this process is restoration. If successful, “you have gained your brother.” Step 1 is to “tell him his fault” privately.

18:16 if he will not hear. I.e., if he remains impenitent, follow step 2: “take with you one or two more,” to fulfill the principle of Deut. 19:15.
18:17 tell it to the church. If he still refuses to repent, step 3 requires that the matter be reported to the whole assembly (v. 17)—so that all may lovingly pursue the sinning brother’s reconciliation. But failing that, step 4 means that the offender must be excommunicated, regarded by the church as “a heathen and a tax collector” (see note on 5:46). The idea is not merely to punish the offender, or to shun him completely, but to remove him as a detrimental influence from the fellowship of the church, and henceforth to regard him as an evangelistic prospect rather than as a brother. Ultimately, the sin for which he is excommunicated is a hard-hearted impenitence.
18:18 bind on earth … bound in heaven. See note on 16:19.
MacArthur, John Jr: The MacArthur Study Bible. electronic ed. Nashville : Word Pub., 1997, c1997, S. Mt 18:15