The following is an excerpt from The MacArthur New Testament Commentary on 1 Peter 3.

You husbands in the same way, live with your wives in an understanding way, as with someone weaker, since she is a woman; and show her honor as a fellow heir of the grace of life, so that your prayers will not be hindered. (1 Peter 3:7)

In the same way refers again to the duty of submission (2:13, 18; 3:1). This time it is the believing husband who submits to serve his wife. Husbands obey that duty by adhering to three basic responsibilities in caring for their wives’ needs: consideration, chivalry, and companionship.

First, husbands are to live with their wives in an understanding way, which means they must be considerate. Understanding speaks of being sensitive and considering the wife’s deepest physical and emotional needs. The word translated live (sunoikountes) means “dwelling together” and refers to living with someone in intimacy and cherishing them. Believing husbands must constantly nourish and cherish their wives in the bond of intimacy:

Husbands, love your wives, just as Christ also loved the church and gave Himself up for her, so that He might sanctify her, having cleansed her by the washing of water with the word, that He might present to Himself the church in all her glory, having no spot or wrinkle or any such thing; but that she would be holy and blameless. So husbands ought also to love their own wives as their own bodies. He who loves his own wife loves himself. (Eph. 5:25–28; cf. Prov. 5:18–19; 1 Cor. 7:3–5)

A believing husband should also be chivalrous to his wife, realizing she is someone weaker, since she is a woman. Just as submission does not imply inherent inferiority for the ones who submit, so the word weaker does not mean the wife is intrinsically weaker in character or intellect than her husband. The word (rendered “weaker vessel” by the King James and New King James translators) also does not mean that women are spiritually inferior to men (cf. Gal. 3:28). It just means that women generally possess less physical strength than men. With that in mind, Christian husbands are the sacrificial providers and protectors of their wives (cf. 1 Sam. 1:4–5; Eph. 5:23, 25–26; Col. 3:19; 1 Tim. 5:8), whether or not the wives are believers.

Third, the husband is to be a companion for his wife as a fellow heir sharing in the grace of life, which refers not to eternal life, but to the true and intimate friendship that belongs only to those who are possessors of God’s most blessed gift in this life—marriage. Peter labels marriage the grace of life because grace (charis) means “unmerited, undeserved favor” (cf. Rom. 1:5; 3:24; 5:15, 17; 12:3; 15:15; 2 Cor. 8:1; 9:8; Gal. 2:9; Eph. 2:7; 3:2, 7; 4:7; 4:29; 2 Tim. 1:9; Heb. 4:16; James 4:6). Marriage is a divine providence given to man regardless of his attitude toward the Giver. Intimate companionship in marriage, the richest blessing of this life, was a foreign concept to the Greco-Roman culture of Peter’s day. Husbands were generally uninterested in friendship with their wives, expecting them to merely maintain the household and bear children. In contrast, the Christian husband is to cultivate all the richness God designed into the grace of marriage by showing honor to his wife in loving consideration, chivalry, and companionship. So that his prayers will not be hindered is the reward God promises to the loving, caring husband (cf. Ps. 66:18; Isa. 59:2; John 9:31; James 4:3). The prayers in view may be specifically for the salvation of an unbelieving wife, but nothing in the text limits it to that. The warning is clearly given that if a husband in Christ is not fulfilling his responsibilities toward his wife, God  may not answer his prayers. No more serious divine threat could be given to a believer than that—the interruption of all the promises of prayers heard and answered (cf. John 14:13–14). That is severe, cutting off the divine blessing, which shows how critical is Christian husbands’ loving care of their partners in this grace of life.

The key to having a positive witness to an unsaved spouse is living an exemplary Christian life as a faithful, submissive spouse. That obedience pleases God and provides the testimony that honors Jesus Christ before the unsaved partner.