Matthew 22:37-40 New International Version
37 Jesus replied: “‘Love the Lord your God with all your heart and with all your soul and with all your mind.’ 38 This is the first and greatest commandment. 39 And the second is like it: ‘Love your neighbor as yourself.’ 40 All the Law and the Prophets hang on these two commandments.”
In the 1984 movie, The Karate Kid, Mr. Miyagi (played by Pat Morita) tells his student Daniel (played by Ralph Macchio) who still has much to learn about karate, "Daniel-san, must talk. Walk on road, hm? Walk left side, safe. Walk right side, safe. Walk middle, sooner or later, [makes squish gesture] get squish just like grape. Here, karate, same thing. Either you karate do 'yes', or karate do 'no'. You karate do 'guess so', [makes squish gesture] just like grape. Understand?"
I've never tried to learn karate, nor to compete in a tournament against others like Daniel was planning on doing. I have finished five 50k trail races, with another one coming up very soon, and anyone who has done ultra-running can see the point Miyagi is trying to make: some things in life can't be attempted with half measures. I can't imagine trying to run 31 miles without training, without the right gear and fuel along the way, "squish just like grape" sounds about right though.
Christianity requires an even more single-minded, all-in, nothing-held-back, level of commitment than anything else you or I may attempt to accomplish in life. There is no room for split allegiances, no allowances for putting other goals and priorities above our devotion to God.
The second commandment, as listed by Jesus, puts our devotion to God in its full context. When we give God our whole heart, soul, and mind, it doesn't eliminate or even diminish our obligations to other people, it actually enables them. God requires that we treat everyone with love because God is the maker of us all and the redeemer of humanity. We are not allowed to put labels on individuals or groups of people and thus exempt ourselves from loving them (see the Parable of the Good Samaritan for a powerful rebuke of those who think 'neighbor' is limited).
Which brings us to 'Christian' Nationalism which runs afoul of both the greatest and the second greatest commandments. {It isn't the only 'ism' guilty of this flaw} The greatest by diluting our devotion to God, the Gospel, and his Kingdom by putting the interests of a particular country on par with them {If you think 'Christian' Nationalism isn't elevating love of country to this level, you haven't interacted with it in its full form either historically or in America today}, and the second greatest by invariably treating our country's citizens as more worthy of love than 'them'. "America First!", whatever else it may be, cannot be a Christian slogan, nor one which God would approve, as it inherently treats people who don't posses American citizenship as lesser than those who do.
Go ahead and love your country, if you think it is the best country to live in your faith won't have any problem with that opinion. But don't, if you wish to obey the commands of Jesus, act as if Jesus mentioned devotion to country when he spelled out the greatest commandment, and don't commit the sin of treating 'them', whoever you may put in that category, as anything less than your neighbor.