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I would assume he was reading Ephesians 4:

1As a prisoner for the Lord, then, I urge you to live a life worthy of the calling you have received. 2Be completely humble and gentle; be patient, bearing with one another in love. 3Make every effort to keep the unity of the Spirit through the bond of peace. 4There is one body and one Spirit—just as you were called to one hope when you were called— 5one Lord, one faith, one baptism; 6one God and Father of all, who is over all and through all and in all.

Romans 14, which addresses denominational variances:

Accept him whose faith is weak, without passing judgment on disputable matters. 2One man’s faith allows him to eat everything, but another man, whose faith is weak, eats only vegetables. 3The man who eats everything must not look down on him who does not, and the man who does not eat everything must not condemn the man who does, for God has accepted him. 4Who are you to judge someone else’s servant? To his own master he stands or falls. And he will stand, for the Lord is able to make him stand.
5One man considers one day more sacred than another; another man considers every day alike. Each one should be fully convinced in his own mind. 6He who regards one day as special, does so to the Lord. He who eats meat, eats to the Lord, for he gives thanks to God; and he who abstains, does so to the Lord and gives thanks to God. 7For none of us lives to himself alone and none of us dies to himself alone. 8If we live, we live to the Lord; and if we die, we die to the Lord. So, whether we live or die, we belong to the Lord.

9For this very reason, Christ died and returned to life so that he might be the Lord of both the dead and the living. 10You, then, why do you judge your brother? Or why do you look down on your brother? For we will all stand before God’s judgment seat. 11It is written:
“ ‘As surely as I live,’ says the Lord,
‘every knee will bow before me;
every tongue will confess to God.’ “[a] 12So then, each of us will give an account of himself to God.

13Therefore let us stop passing judgment on one another. Instead, make up your mind not to put any stumbling block or obstacle in your brother’s way. 14As one who is in the Lord Jesus, I am fully convinced that no food[b] is unclean in itself. But if anyone regards something as unclean, then for him it is unclean. 15If your brother is distressed because of what you eat, you are no longer acting in love. Do not by your eating destroy your brother for whom Christ died. 16Do not allow what you consider good to be spoken of as evil. 17For the kingdom of God is not a matter of eating and drinking, but of righteousness, peace and joy in the Holy Spirit, 18because anyone who serves Christ in this way is pleasing to God and approved by men.

19Let us therefore make every effort to do what leads to peace and to mutual edification. 20Do not destroy the work of God for the sake of food. All food is clean, but it is wrong for a man to eat anything that causes someone else to stumble. 21It is better not to eat meat or drink wine or to do anything else that will cause your brother to fall.

22So whatever you believe about these things keep between yourself and God. Blessed is the man who does not condemn himself by what he approves. 23But the man who has doubts is condemned if he eats, because his eating is not from faith; and everything that does not come from faith is sin.

There is another passage specifically on the issue that differing beliefs on non-core gospel issues will arise, to believe such “fervently in your heart” but to not let it interfere with the core gospel.