concordia

1 Peter 3:15 – But in your hearts revere Christ as Lord. Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect.

Harmony and Concord are the same.  Concord means “having the same heart”.  People who have in their hearts revered Christ as Lord should have the same heart.  So why all the strife among us?  Here is what Luther said.

Harmony is the imperative virtue for the Christian Church. Before the other virtues — love, meekness — can be manifest, there must first be concord and unity of heart among all. It is impossible that outward circumstances of human life be always the same; much dissimilarity in person, station, and occupation is inevitable.
To this very unlikeness and to the natural depravity of flesh and blood is due the discord and disagreement of men in this world. Let one become conscious of personal superiority in point of uprightness, learning, skill or natural ability, or let him become aware of his loftier station in life, and he immediately grows self-complacent, thinks himself better than his fellows, demands honor and recognition from all men, is unwilling to yield to or serve an inferior and thinks himself entitled to such right and privilege because of his superiority and virtue.
4. Pride is the common vice of the world, and the devil fosters it among his numerous followers thereby causing every sort of misery and unhappiness, corrupting all ranks and stations, and rendering men vicious, depraved and incapable of executing good. In opposition to this vice the apostles diligently admonish Christians to be of one mind, regardless of station or occupation, since every individual must remain in the position to which he has been ordained and called of God. All ranks and stations cannot be one.
Particularly is this true in the Church; for in addition to the outward difference of person, station, and so on, there are manifold divine gifts unequally distributed and varyingly imparted. Yet these many dissimilarities, both spiritual and secular, are to be amenable to the unity of the spirit, as Paul calls it, or a spiritual unity. Just as the members of the physical body have different offices and perform different functions, no one member being able to do the work of the other, and yet all are in the unity of one bodily life; so also Christians, whatever the dissimilarity of language, office and gift among them, must live, increase and be preserved in unity and harmony of mind, as in one body.
5. This matter of harmony is the first and most necessary commandment enjoined by the doctrine of faith; ay, this virtue is the first fruit which faith is to effect among Christians, who are called in one faith and baptism. It is to be the beginning of their Christian love. For true faith necessarily creates in all believers the spirit that reasons: “We are all called by one Word, one baptism and Holy Spirit, to the same salvation; we are alike heirs of the grace and the blessings of God. Although one has more and greater gifts than another, he is not on that account better before God. By grace alone, without any merit of ours, we are pleasing to God. Before him none can boast of himself.”
6. How can I think myself better than another by reason of my person or my gifts, rank or office? Or what more than I has another to boast of before God concerning himself? No one has a different baptism or sacrament, a different Christ, from mine, or grace and salvation other than I have. And no individual can have another faith than have Christians in general, nor does he hear any other Gospel or receive a different absolution, be he lord or servant, noble or ignoble, poor or rich, young or old, Italian or German. When one imagines himself different from or better than his fellows, desiring to exalt and glorify himself above others, he is truly no longer a Christian; because he is no longer in that unity of mind and faith essential to Christians. Christ with his grace is always the same, and cannot be divided or apportioned within himself.
7. Not without reason did the beloved apostles urge this point. They clearly saw how much depends upon it, and what evil and harm result from disregard of the commandment. Where this commandment is dishonored, schisms and factions will necessarily arise to corrupt pure doctrine and faith, and the devil will sow his seed, which afterwards can be eradicated only with difficulty. When once self-conceit rules, and one, pretending more learning, wisdom, goodness and holiness than his fellows, begins to despise others and to draw men to himself, away from the unity of mind which makes us one in Christ, and when he desires the first praise and commendation for his own doctrine and works, his own preaching, then the harm is already done; faith is overthrown and the Church is rent. When unity becomes division, certainly two sects cannot both be the true Church.