In Romans 14, Paul identifies one cause of difference as being due to the strength/weakness of faith. He anticipates in Rome's church that people will look at the cleanness of food and the specialness of certain days differently. All food is clean and all days are holy for someone who follows Jesus, but some have weaker faith and don't embrace that freedom.
Principle 1. Welcome as God welcomed you (neither the weak or the strong in faith should judge the conservatism or liberty of the other).
Paul addresses how they welcome one another - 14:1-3. Knowing God's welcome of someone else (and yourself) defines your welcome of others. A strong faith appreciates God's welcome deeply, and is surely exponentially welcoming.
We can want to keep everyone in line - but people aren't meant to live for people. We live for Jesus. And you need to get what you do right with him... which can look deeply contradictory as one person's read on what will honour Jesus isn't quite the same as another person. The question is - what can I do with thanksgiving, what can I do worshipfully... for one they eat, another doesn't... one keeps a day special, another doesn't... and so on. Messy! The heart matters more than the outward action.
Some things are objectively right and others wrong. And Paul stands very confident for himself....
Yet, in the same breath, Paul takes the conscience very seriously. He who says he is 100% sure that foods are clean (Jesus said so!) values personal thinking and conscience very strongly. If your faith is "weak" so that you can't bring yourself to fully live in the freedom you have in Christ, that's not something to take lightly. It's really serious:
Principle 3. Certainty doesn't make you dogmatic, it makes you flexible (gospel-depths mean you're less shockable, less worries about other people, more self-forgetful).
Paul has deep convictions about what he believes. He is convinced. Certainty doesn't make for dogmatic approaches... Paul sees that certainty leads to flexibility. And the stronger the certainty the more flexible we should be - especially on things that aren't 'gospel issues'.
e.g. The Kingdom is not a matter of food and drink... so don't fall out over food and drink. Don't make something secondary into a headline... don't divide over something that you can viably flex about. Ask what's good for the gospel more than what do I want. Ask what really matters and what really shouldn't.
In matters of unity then....
Let's embrace the ambiguity and messiness of accomodating one anothers consciences. That'll draw some flak, it'll look messy and inconsistent, but guarding the gospel means guarding people's consciences.
Let's give room for faith to grow rather than destroying one another in a quest for neatness and people-pleasing.