I'm the sort of person who bangs a drum for Christ in the OT. Specifically I think it's important to maintain that knowledge of God is always in Christ.
In my experience there are three ways to do this:
1) From the OT forwards
Basically you point out where the OT reveals an Appearing LORD, the Angel of the LORD, the Commander of the LORD's army etc. And you say 'Look - there He is.'
2) From the NT backwards
Basically you show how Jesus and the Apostles just assumed that the OT saints knew Christ.
Here you point out how Christ is the Image, Word, Way, Truth and Life of God - and always has been.
Typically I encounter these kinds of resistance.
Against 1) I tend to find an underlying assumption something like:
The OT saints could not have even a proto-trinitarian understanding of deity in distinction among multiple Persons called 'LORD'. They (therefore) could not have an understanding of the distinct Person of Christ.
Of course there is no Scriptural warrant for this. It is just an assumption. But a very strong one. Quite rarely will this assumption yield even when confronted with a barrage of OT texts.
If it is finally conceded that Christ in His distinct deity is present in the text, the interlocutor will typically say "Ah but nobody at the time realised it."
Again this is not a Scriptural assertion, but it has tremendous force on your interlocutor. They may even argue as though "They spoke better than they knew" was in the bible. It's not in the bible. And it's an extremely poor assessment of the bible's teaching on OT faith and prophecy.
Against 2) I find an underlying assumption something like:
Whenever Jesus and the NT speak of Christ-focused faith and experience in the OT, they are always re-reading the OT in a way that was neither intended nor grasped by the OT saints.
Here's a frustrating little quirk! Bring up a barrage of NT texts that say something like "Seeing what was ahead David spoke of the resurrection of the Christ" (Acts 2:31) and you will receive a reply something like "Ah yes, Peter said that, but only with Pentecostal eyes of faith."
Um... [scratch head]... so David saw and spoke of Christ and His resurrection but... only once Pentecost happened? ...?
Against 3) I reckon the underlying assumption is something like:
You don't really need to know Jesus to know God. Obviously it's best. But not essential.
To be honest I think this is the one that really throbs beneath much of these discussions.
I don't mind if people have false assumptions 1) or 2). I'm fairly optimistic that eventually the Scriptures will do their work here. I find false assumption 3) to be the most intransigent and the most worrying.
Go here to read quotes from history firmly opposed to such an idea.
Go here to read a debate we're having on just this topic - we're up to 67 comments!
And keep reading the blog - because that's what 'Christ the Truth' is all about.