Here John is speaking of his ministry. He is saying that he has been appointed to his ministry by God. This verse is not speaking of God's grace.
Indeed, God has to give it to you before you can accept His offer of grace. But ‘accepting’ is not ‘taking’ or ‘stealing’, but: receiving what has been offered to you.
God gave His Son, so your sins can be forgiven and you can receive eternal life if you ask Him for forgiveness and believe in Him. He is offering you His grace, as a gift. You can be thankful for this free gift.
When you are given a gift, you must receive it (accept the sacrifice of the Lord Jesus) and take hold of it (believe it is given to you personally). Then, you should not leave the gift on a shelf, but unwrap it and use it for its intended purpose (only by living in a personal relationship with God, you will experience what you believe).