The Gospel of the Kingdom was given to the Jews, and it was under the law. More was required than just faith - works of the law.
Jesus Christ willingly went to the cross and died for our sins for big reasons. Jesus Christ himself revealed the Gospel of the Grace of God to the Apostle Paul, and it included what changed after the Cross.
Titus 3:4-7 KJV But after that the kindness and love of God our Saviour toward man appeared, 5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost; 6 Which he shed on us abundantly through Jesus Christ our Saviour; 7 That being justified by his grace, we should be made heirs according to the hope of eternal life.
1 Corinthians 15:1-4 KJV 1. Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand; 2. By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain. 3. For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures; 4. And that he was buried, and that he rose again the third day according to the scriptures:
I love the old hymn - Jesus Paid it All - all to Him I owe. We are now saved by faith through God's matchless Grace - Grace that we can't earn and will never deserve. Faith comes by hearing and hearing by the Word of God. It's a simple, childlike faith in the death, burial, and ressurection of Jesus Christ. Salvation is not on the installment payment plan, nor is it by works that we can do. Jesus Christ did Pay it All at the Cross, and we have nothing worthy to add to His perfect and completed work at the Cross.
Ephesians 2:8-10 KJV For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: 9 Not of works, lest any man should boast. 10 For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them.
We accept God's unspeakable Gift through simple faith plus nothing. God's Word tells us there are many things we SHOULD do after we are Saved, but we must know those things are not required for Salvation or to maintain Salvation. There are additional rewards for good works if they are done for the right reasons (i.e. love for Jesus Christ out of a heart of appreciation). I give thanks that Salvation is simple enough for a child to understand.