Yes, God not only would take you back -- but He wants to take you back! He loves you even more than your grandfather did and yearns to welcome you into His family forever.
Have you ever asked yourself why you decided to leave God out of your life? I suspect I know the reason: You wanted to be free to run your own life, and you didn't want God to interfere with it. But has this brought you true happiness? To put it another way, can you honestly say that your life is better than the faith-filled one your grandfather led? I doubt it. The Bible says, "The fear of the Lord leads to life: Then one rests content, untouched by trouble" (Proverbs 19:23).
As I read your letter, I couldn't help but think of Jesus' parable of the prodigal son. (You can find it in Luke 15:11-32.) In it, Jesus told about a son who turned his back on his father, leaving home and spending his inheritance in wild living. But in time he came to his senses, and when he returned home his father ran to welcome him. This is a vivid picture of God's love for us.
Don't turn your back on God any longer, but repent of your sins and commit your life to Jesus Christ. Then thank Him not only for forgiving you, but also for giving you the gift of eternal life with Him -- and with your grandfather -- in heaven forever.