Answer
Salam Dear Reader,
Thank you very much for your question and for contacting Ask About Islam.
First of all, we need to understand that the Gospels, which purport to convey to us the teachings of Jesus were subject to changes and modifications over time. The following factors are worthy of note here:
The Gospels are compilations, made decades after Jesus, and were written mostly by people who had not met Jesus in person. Of course, the most widely accepted Christian position is that the Gospels, written by his disciples, were by divine inspiration and they are therefore true and contain no error.
We see the names of the four Gospel writers among the names of the twelve disciples of Jesus, with the exception of Luke. But the chances of the Gospels being the writings of any of the twelve apostles of Jesus are very remote, as mentioned by many Christian scholars themselves.
There is much evidence in the Gospels, pointing to this fact. After Jesus’ era, there was a lot of confusion and there were conflicting beliefs about Jesus and his Gospel. This is quite clear from what Luke writes at the beginning of his Gospel, for instance:
Luke (1: 1–4):
‘For as much as many have taken in hand to set forth in order a declaration of those things, which are most surely believed among us,
Even as they delivered them unto us, which from the beginning were eyewitnesses, and ministers of the word;
It seemed good to me also, having had perfect understanding of all things from the very first, to write unto thee in order, most excellent Theophilus,
That thou mightest know the certainty of those things, wherein thou hast been instructed.’
A close study of the above selection brings into focus the following facts:
- Many people have written about the story and the teachings of Jesus.
- These ideas were delivered to Luke by eyewitnesses; which means that Luke the Gospel writer was not an eyewitness.
- Luke’s statement: ‘It seemed good to me also’, suggests that he was not writing under divine inspiration; nor does he claim so.
- ‘… to write to thee in order, most excellent Theophilus …’ shows that this is intended as a letter addressed to one Theophilus. So this is further evidence that Luke’s account cannot be the word of God, addressed to the whole humanity.
We may say that the Gospel accounts contain the words of Jesus, as reported by the gospel writers, on hearsay. Consequently, there are bound to be errors in them, and most certainly they reflect the personal beliefs of the writers themselves. 
The foregoing means that Muslims need not implicitly believe in the words of Jesus, as given in the present Gospels, as being his exact words.
We read in the Gospel, according to Matthew (5: 39):
‘But I say unto you that ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.’
As you yourself have said, this teaching is not practical. Also remember in this context what Prophet Muhammad (peace be upon him) instructed us:
‘When you see an evil act, you have to stop it with your hand. If you can’t, then speak out against it with your tongue. If you can’t, then at least you have to hate it with all your heart. And this is the weakest of faith.’
(Reported in The Collection of Hadiths of the Prophet by Imam Muslim.)
If these words were the words of Jesus we appreciate them. Yet, Islam came with a balanced view between the rigid jurisprudence of Judaism and the sole focus on ethics and morals of Christianity. That is why Islam is called the straight path and the Quran is called the mizan (balance).
Islam also has a very strong ethical content, regarding mercy, as always recommended on the personal level. It doesn’t however tolerate injustice or accept aggression. Muslims are also required to seek justice and to defend the vulnerable and the powerless. They should never take the stance of indifference or neutrality in such cases.
We believe that the message of Jesus was meant to soften the rigid laws of Jewish jurisprudence and that was a very relevant task at its time. Muslims do believe in the previous messages and do not distinguish between prophets.
Still, we also have a distinct and comprehensive shari`a (Islamic law) that includes manners, ethics social logic and principles, as well as rules that apply to economics and politics.
Muslims do not see Islam as detached from former divine messages, and the jurists have set the rule that if there is a space of overlapping instructions, then Muslims might benefit from the commandments of former messages. Such commandments came to bring people to the path of Allah.
Jurists has set the condition that this interaction and mutual ‘dialogue’ would not contradict with a basic rule in Islam, nor a verse in the Quran or an authentic hadith (saying) of the prophet (peace be upon him).
Resisting evil with all our might is a responsibility Muslims carry, rather than yielding to it – no doubt. For this reason, we would set the teaching ‘resist not evil’ as a moral instruction that suited the purpose of that message at a specific historical moment. The message of Islam is the final message, which has a universal nature.
Thus, the Quran is the last testament, which sets the rule that mercy has to be accompanied by seeking justice, and they cannot be separated.
As for your question: ‘… isn’t it wrong to disagree with the teachings of the messengers of Allah?’ The answer is: It is wrong on our part to disagree with the teachings of the messengers of Allah. But we must be sure we know what those teachings were and who they were taught to.
We need to avoid taking Jesus’ teachings as if each phrase can be treated in isolation and as completely general. Given the doubts that surround much of his teachings today, we need cautious and careful analysis of many sayings to reach a good evaluation of his teachings.
Taking a single phrase out of the context of who he was instructing and out of the context of what else was being taught to them, by Jesus and others, will yield many problems and much confusion. Even a subtle change of wording between what he said and what was actually reported may make a great deal of difference – and we haven’t even mentioned the problems of translation! 
If we take for example the idea of “resist not evil”, this needs to be understood within the context where the Jews were excessively unforgiving and seeking retribution for every evil done. What is known as “the Lord’s prayer” to Christians illustrates this point well, with its emphasis on “forgive us our trespasses as we forgive those who trespass against us”. Much of Christ’s message was about this.
The Quran sets the record straight and confirms the clear and balanced moral rule in this area, which Jesus was trying to bring his people back to:
{We ordained therein for them: “Life for life, eye for eye, nose or nose, ear for ear, tooth for tooth, and wounds equal for equal.” But if any one remits the retaliation by way of charity, it is an act of atonement for himself. And if any fail to judge by (the light of) what God hath revealed, they are (No better than) wrong-doers.} (Surah 5 Verse 45)
{Nor can goodness and evil be equal. Repel (evil) with what is better: Then will he between whom and thee was hatred become as it were thy friend and intimate!} (Surah 41 Verse 34)
And Allah knows best.
Salam.