The abundant life that Jesus came to give you is that God loves you and he’s never going to stop.
It’s that all your sins are forgiven and all the things that used to separate you from God are now gone, and now, because of Jesus — because he died for you and was raised from the dead for you — there is not a single thing in the universe that can stand in the way of God’s grace to you.
It is unending grace. God will never get tired of you. God will never regret saving you. God’s mercy will flow into your life as an endless ocean of inexhaustible wonder. So brain tumors, come what may! You are never going to die. You are never going to be snatched from his hands. You are never going to be in a situation where he is not already there, with you, and working for you, so that all of your suffering, and all of your pain, and all of your circumstances are used by him to cause good for you.
And because that is true, when you’re standing on shaky ground, or when it feels like the mountains are being swallowed by the sea, or your sinking up to your neck in the swamp of sorrow, you can know that God is not mad at you. He calls you his child. He calls you by name.
You are so alive. You know that in him all the things that you have ever really wanted are truly found; the deepest cravings of your soul are satisfied in his glory. You become the richest, most secure person in the world because you have the greatest of all gifts and it will never ever be taken away from you. He loves you and he’s never going to stop.
That is the abundant life. And Jesus said he came to give that to you. He is the door to that.