Of which salvation the prophets have enquired and searched diligently, who prophesied of the grace that should come unto you: Searching what, or what manner of time the Spirit of Christ which was in them did signify, when it testified beforehand the sufferings of Christ, and the glory that should follow. Unto whom it was revealed, that not unto themselves, but unto us they did minister the things, which are now reported unto you by them that have preached the gospel unto you with the Holy Ghost sent down from heaven; which things the angels desire to look into. (1 Peter 1:10-12 KJV)
Here I see that the Old Testament and the New Testament are intertwined. This passage makes me think of Hebrews 1:1.
God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds; (Hebrews 1:1-2 KJV)
The Old Testament prophets and the children of Israel had the types and shadows of the Saviour to come, His suffering and sacrifice on our behalf for our sin. They had the tabernacle and the sacrifices and rituals which all pointed to Christ. They accepted by faith that there was a coming Saviour. Hebrews 11 attests to their faith.
They looked forward in faith to Christ's sacrifice while we look back in faith to His atoning sacrifice. We are all saved by faith in Christ's atoning sacrifice on our behalf. To throw out the Old Testament as being obsolete is a huge mistake that some make. We need the whole of both sections of the Bible or we miss out on a lot. It is one package deal that shouldn't be separated.