We preachers are rightly urged to preach Christ from every Old Testament passage. This is sound advice, and finds support from our Lord himself, in Luke 24.
There are, however, some dangers and pitfalls in doing so, as helpfully pointed out by Colin Adams. Here’s what he has to say (in his own words, but slightly abbreviated):
i) Christ is everywhere in the Old Testament but he’s also the promised one.
Christ was certainly present in the entire history of redemption (John 8:56-57, Hebrews 11:26, Jude 5). He was active in creation, in the exodus and in the time of Israel’s exile.
Yet in the Old Testament he is predominantly presented as the promised one (Genesis 3:15, 49:8-12, Numbers 24:17, 2 Samuel 7)…His appearance in the New Covenant era is climactic (Galatians 4:4, 1 John 3:8, Hebrews 1:1-2). Christ is consistently revealed in the Bible, but also increasingly revealed. So don’t rush too quickly from the Old to the New…
ii) Christ is everywhere in the Old Testament but don’t neglect the Trinity.
Sometimes evangelical preachers sound more like Modalists than Trinitarians. When we preach the Old Testament – and even when we bring its teaching forward to the age of fulfilment – we should note that it reveals not just the Son, but the Godhead. Christian writers in recent years have pointed out the dangers of an exclusive focus on the Son. We do not honour Christ when we squeeze the Father and Spirit out of our preaching…
iii) Christ is everywhere in the Old Testament but there are moral examples too.
Preaching that is heavy on ‘to do’ has been given a bad wrap in some circles. Detached from Christ, such preaching leads to the hubris of self-help or the despair of self examination. But ‘preaching the law’ so to speak (when properly done) can both lead us to Christ and be a response to his grace. Further, the New Testament sometimes uses the Old Testament to either warn us of sin or give an example of godliness (1 Corinthians 10:1, James 5:17, Hebrews 11, 12:1).
iv) Christ is everywhere in the Old Testament but beware of artificial links.
The five stones that David picked up to slay Goliath do not represent the five books of the Pentateuch… which represent the law that is fulfilled in Christ…which then matches up with the five teaching blocks that are found in Matthew’s gospel. This is an extreme example, but it highlights the dangers of allegorizing and making unlikely links. Only by steeping ourselves in Scripture will we develop a greater instinct for what is a legitimate connection and what is just arbitrary and fanciful.