In order to have unity, we must be agreed. I have found over the years, that the best place to find unity with other believers is through the word of God. It is impossible to agree with everyone on every single point, so we must determine what are essentials and what are non-essentials of the faith.
An essential belief would be, who do we think Jesus is, salvation only through the cross, or you must be born again to enter the kingdom of heaven.
Non-essentials, would be our belief on Spiritual gifts, or when the Lord is going to come back, pre, mid or post. Though many non-essentials are important, they should never be issues for division or removing ourselves from fellowshipping with other believers.
The sad thing that is currently happening in the body of Christ, is that we are making non-essentials essential. There are things in the Bible that are not essential so as to determine where we will spend eternity, but we have at times turned them into such.
For example we have had those fellowship with us in the past who have told others in our fellowship, unless they speak in tongues they are not saved. This would be making a non-essential, essential.
There could be a dozen more things we could touch upon, that would fit this criteria.
I have found that even with the essential doctrines, it is still really important that we walk in love toward those who reject, and to treat them with a gentleness and respect as we discuss these essential doctrines.
If we really care for their souls, our desire should not be just to win an argument, but speaking the truth in love, will hopefully gain our brother and sister,[potentially] and so cover a multitude of sins. Lets always keep the doors open, to our atheist friends, and those who feel we are silly for what we believe, lets stay respectful towards all, in the hopes that we may yet win some!