"Accounting that God was able to raise him up, even from the dead; from whence also he received in him a figure." (Hebrews 11:19 KJV)
"But tell me this - since we preach that Christ rose from the dead, why are some of you saying there will be no resurrection of the dead? For if there is no resurrection of the dead, then Christ has not been raised either. And if Christ has not been raised, then all our preaching is useless, and your faith is useless. And we apostles would all be lying about God - for we have said that God raised Christ from the grave. But that can't be true if there is no resurrection of the dead. And if there is no resurrection of the dead, then Christ has not been raised. And if Christ has not been raised, then your faith is useless and you are still guilty of your sins. In that case, all who have died believing in Christ are lost! And if our hope in Christ is only for this life, we are more to be pitied than anyone in the world." (1 Corinthians 15:12-19 NLT)
If you really don't understand the term, "reckon," it isn't really that difficult to see Paul's reckoning of what might be if it was true that Christ didn't raise up from the grave; what pitiful life it would be for all of us that believe, if that was really the case. Thankfully, we are assured that He did, because it can be easily proven, if we clearly believe in the written Word of God!
I really don't know how else to say this, except to say that somehow the believer's in God, from a very long time before Jesus came, actually believed that He was coming to save the world, and that He was going to be killed, and that God would raise Him back up from the dead. Other than that, why would Abraham even consider it possible, except for the fact that God said his son would have many children?
Here is the thing that made be sure, surer of who Jesus is, was, and always will be: By truly believing what the Bible says, from the beginning to the end, it is all very plain to see; if that is, you're willing to believe! I suppose that if God had made everything through His Son, which is the Word, then I also figure that He is also God! It is really undeniable to reckon that is true...
"In the beginning the Word already existed. The Wood was with God, and the Word was God. He existed in the beginning with God. God created everything through him, and nothing was created except through him." (John 1:1-3 NLT)