Tim Keller is fond of saying,
Religion operates on this principle:
If I obey, then I will be accepted.
Christianity operates on this principle:
I am accepted, therefore I obey.
There are two ways to approach God; either through religion, or through the gospel; either through my performance, or through Christ's performance.
Similarly, there are two ways to approach marriage.
There is religion marriage, and then there is gospel marriage.
In a religion marriage, each partner operates on the following premise.
If you change your behavior, I will accept you.
In religion marriages, people make rationalizations like these;
"I will belittle my husband because he's cold and distant." "I will withhold love from my wife because she belittles me."
In a religion marriage, each partner refuses to change first. Each is wholeheartedly committed to taking the second step. A religion marriage thus creates a closed loop of relational destruction. And as the relationship worsens, each partner must create more justifications for their transparently deplorable behavior. They do this by (1) demonizing their spouse ("he's the worst husband in the world!"; "I'd rather be married to anyone but her!"), or (2) self-victimization ("no one has it worse than me"). Religion marriages make people prideful, callous, resentful and self-absorbed.
A gospel marriage, by contrast, operates on an entirely different premise:
I accept you, and trust that acceptance changes behavior.
In a gospel marriage, each person embraces the love of God in Christ. They abide in his love (John 15:9). It is their very life (John 15:5). As a result, they can love their spouse with no strings attached. Their love isn't manipulative (e.g., "if I do nice things for him, he better do nice things for me!"). In a gospel marriage, each partner is motivated - first and foremost - by God's love; not their spouse's. They love out of grateful response to God's revealed love, not to gain greater control over their spouse's behavior.
Yet, each partner trusts that God can and will use their love to change their spouse. Kindness leads to repentance (Romans 2:4). God won us with kindness. Spouses win each other the same way.
Gospel marriages make people more joyful, more self-forgetful, more generous, and more loving. In short, they make people more like Jesus.
I'm so thankful that my wife is an avid practitioner of gospel marriage. She shows me absurd amounts of grace. And I can honestly say that - aside from Jesus - nothing has motivated me to love my wife more than her mercy towards me.
I realize that I'm presenting two extremes. Most marriages fall somewhere in between religion and gospel. Thankfully, our Savior is a perfect spouse, whose perfect love will purify us forever (Ephesians 5:25-27; Revelation 19:6-9). In the meantime, reflect on his love for you, and then love your spouse in radical ways.