The most obvious answer is that truth is a means to dominate and control people, especially people who consider something else to be true. For Christianity this would be the utility that truth of doctrine would have for preachers, bishops, and other administrators in Christ. For Christians this still leaves open the question of why allowing oneself to be administered and controlled is felt to be a good thing. The pleasant feelings of doing “good” by submitting to church authority figures can be explained in terms of internalized authority: one performs “praise-worthy” actions and it is as if the authority figures themselves praise one as a good Christian. For administrators in Christ it is a question of power and the submission of their flocks to God’s specially appointed authorities. One need only consider the sense of power that would enable any priest to feel entitled to abuse of trust and position that led to sexual abuse scandals in the Catholic Church.
Neither truth as political expediency nor truth as principled swindle answers questions of the uses (and presumably abuses) of truth for Christians on the ground, as it were. Belief in the possession of truth by oneself and by one’s fellows creates a sense of group identity and cohesion. There is an “us” and there is a “them.” This happens regardless of the truth or falsity of a group’s beliefs. Belief in the truth of one’s doctrines contributes to a stronger sense of belonging to the Church. The greater the discipline in belief the greater the sense of belonging.
A group that defines itself in terms of beliefs requires the existence of nonbelieving outsiders. “We” are the ones who have the truth, “they” do not. The truth and substance of the beliefs is of no consequence. The in-group is created in reaction to and is dependent on nonbelievers : “we” are not “them.” This dependency on the existence of outsiders implies that if a group of believers is too successful in spreading their beliefs and exhaust their supply of potential new recruits then group cohesion suffers. There are no longer outsiders against which the group can define itself. A new outsider is necessary. Hence the creation of heresies and the hunt for heretics as well as the creation of scapegoats. It is the logic of the concepts enabled by an absence of critical reflection.
It is hardly surprising that in Christianity the concept of truth is absolute and binary: either one has the truth or one does not. Either truth or error. Either one is a Christian or one is not. Either one is a member of the Church or one is not. Doubt is sin or rather doubt is sin only if one takes it seriously and acts on it. Doubt means the danger of no longer being one with one’s fellow believers. Hence the hatred of apostates.
None of the above touches the substance of Christian Doctrine. It could all be true anyway. Nothing undermines veracity and honesty as much as the discovery of social motivations for adherence to one’s beliefs. There is every reason to lie when telling the truth could result in the loss of profession, friends, social status, and the like. Can a Christian ever be trusted to be honest about Christianity? That a Christian does not mean to lie does not make up for a lack of concern with truthfulness and honesty.