The Seal of the Spirit

God does have proof that He owns His people!

In Pilgrims Progress and Revelations this is seen as something actually written on the forehead like a divine cattle brand, Slave-mark or forehead tattoo (now that would be a funny church requirement for membership - all right kids lets line up for the forehead tattoos!) 

This proof is the seal of the Holy Spirit, as described in the passage below. Knowing that we have the Spirit assures us that God owns us (like a Slave), and that He possesses a love for us as His people. The Spirit himself bears witness with our spirit that we are children of God (Rom 8:16). The Father and the Son gift Him (the Spirit- is a "Him" not an "It") to all believers at conversion, for the sealing happens when we hear - heed- the gospel of salvation (EPH 1:13-14).
The Holy Spirit gifted at the moment of faith is our money back guarantee that we will make it to heaven- no genuine faith, no Spirit, no Spirit, no entrance to Heaven. Like a down payment is a promise that the loan will be paid; the Spirit is God's promise that you will see the day of Glorification.
His promise never fails! The blood of Jesus cannot fail to purchase those for whom it was shed, and the Spirit helps us see that His blood has been shed for all believers. Let us pray that we would, by God's Spirit, recognize that we belong to Him forevermore.

EPH 1;13-14
In Him you also, when you heard the word of truth, the gospel of your salvation, and believed in Him, were sealed with the promised Holy Spirit, who is the guarantee of our inheritance until we acquire possession of it, to the praise of His glory.