How can we expect the Spiritually Dead to have understanding of spiritual things? I think that a bit of patience is of order, since we who are His were once in the same place.
My own opinion.
We expect the Spiritually Dead to understand the Gospel this way:
John 16:7-9
King James Version (KJV)
7 Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.
8 And when he is come, he will reprove the world of sin, and of righteousness, and of judgment:
9 Of sin, because they believe not on me;
All divine truth has always been revealed to men, God initiating relationship. The response of men to the revealed will of God is what varies, either rejection or reception. That will has been progressively revealed through the Ages, culminating in the revelation of the Gospel of Christ. The Comforter's Ministry centers on glorifying Christ, and this is what He does when HE brings the natural man under conviction of sin (that we are sinners), righteousness (that we are not and only Christ is), and judgment (great motivation for repentance, as the reality of our separation and fate are shown to us):
John 15:25-26
King James Version (KJV)
25 But this cometh to pass, that the word might be fulfilled that is written in their law, They hated me without a cause.
26 But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me:
(and please bear with me I have not decided on font 5 or 4 to enlarge Scripture. Thinking 4 will do it.)
John 16:12-14
King James Version (KJV)
12 I have yet many things to say unto you, but ye cannot bear them now.
13 Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come.
14 He shall glorify me: for he shall receive of mine, and shall shew it unto you.
And keep in mind this was spoken to Christ's closest disciples. They too would fall under that convicting Ministry. And we see that the Lord states "I have yet many things to say unto you, but
ye cannot bear them now."
The application to the OP's question would be, from my view, that when we witness we rely on that ministry which speaks to the heart, rather than how we normally engage in discussion, which involves the intellect primarily. Even those who know little more than how one is saved can be used to minister the Gospel, and it is understanding that it is not what we say, but how the Comforter impacts the heart of those we witness to that takes the responsibility from our shoulders and places them squarely on God's. So our view of those we consider lost should not necessarily be governed by our view of them, but God's. We understand the basics, and sometimes it will be that someone we witness to, or teach...will be offensive to us. But if we consider how offensive each of us are to Holy God...we might not be inclined to refrain from the task before us, which is...simply to convey the Gospel of Christ.
And most of that is simply being available for God to work through us.
God bless.