BELIEVE IN ... (FILL IN THE BLANK)

It's interesting to step back and take an overview of the word "faith/believe" in the New Testament.

If I am reading my sources correctly, the Greek word "pistis", the noun form translated most often as "faith" occurs 243 times in the New Testament. 

Coincidently, the verb form, "pisteuo", translated typically as "believe" also occurs 243 times. 

Both exist quite often without any object. But when they do have objects, there is quite a range.

Here's a sampling of the noun form, pistis/faith: 

The Bible speaks of faith...

• in God (Mark 11:22)
• toward God (Heb 6:1)
• in our Lord Jesus Christ (Acts 20:21)
• in him who justifies the ungodly (Romans 4:5)
• in the Son of God (Gal 2:20)
• in Christ Jesus (Col 1:4)
• in the truth (2 Thess 2:13)

The verb form, pisteuo/believe, has even more variety in its objects:

The Bible speaks of people believing...

• John the Baptist (Matt 21:32)
• in the gospel (Mark 1:15)
• that what Jesus says will come to pass (Mark 11:23)
• that you have received what you asked for in prayer (Mark 11:24)
• the reports about the risen Christ (Mark 16:14)
• Jesus's words (Luke 1:20)
• all that the prophets have spoken (Luke 24:25)
• in His name (John 1:12)
• the Scripture and the word that Jesus had spoken (John 2:22)
• in the Son (John 3:36)
• that "I am He" (John 8:24)
• in the Son of Man (John 9:35)
• the works Jesus performed (John 10:38)
• that "you are the Christ, the Son of God, who is coming into the world (John 11:27)
• that God sent Jesus (John 11:42)
• in the Light (John 12:36)
• that Jesus came from God (John 16:27)
• that Jesus is the Christ, the Son of God (John 20:31)
• Philip as he preached the good news about the Kingdom of God and the name of Jesus Christ (Acts 8:12)
• in the Lord (Acts 9:42)
• in God (Acts 16:34)
• in Him who raised from the dead Jesus our Lord (Rom 4:24)
• that God raised Jesus from the dead (Rom 10:9)
• that Jesus died and rose again (1 Thess 4:14)
• that Jesus is the Christ (1 John 5:1)
• in the name of the Son of God (1 John 5:13)

But most often, the Bible simply speaks of "believing in Jesus". 

It's funny that when you hear people try to define exactly what a person must "believe" in order to be saved, the answer is usually something like "You must believe that Jesus died for your sins and that He is the only way for you to be forgiven and to get to heaven".

But belief is never formulated exactly that way even once in any of these verses. 

Wouldn't you think that if faith were simply a mental assent to some propositional truth, the magic formula would be repeated throughout the New Testament, so that nobody would miss it?

When you look at the lists above, it seems to me that the emphasis is not at all on propositional truths, per se.

The emphasis is not on the object. The emphasis is on the verb itself.

Believing is a heart attitude. 

Believe = Trust.