I was reading this helpful primer on presuppositional apologetics when I came across this timely quote:

The non-Christian world often judges the value of the gospel by the consistency of life observable in the believer. At church, work, or at home, we render our defense ineffectual by inconsistencies in our lives. On occasion one can hear a Christian defending the faith before an unbeliever and at the same time attacking his Christian brethren with whom he has differences on secondary matters. Such Christians often fail to realize that their outspoken opposition to other believers actually hinders their defense of the faith. In fact, there is hardly a greater obstacle to Christian apologetics than the fighting and bickering which goes on in the church. Jesus showed his concern about the affect of disunity on the church's testimony to the world by saying, "I in them and Thou in Me, that they may be perfected in unity, that the world may know that Thou didst send Me... (John 17:23). We must fill our churches with Christian love and unity if we are to be convincing to a world of disbelief (Richard Pratt, Every Thought Captive: A Study Manual for the Defense of Christian Truth)