“And you also were included in Christ when you heard the message of
truth, the gospel of your salvation. When you believed, you were marked in him
with a seal, the promised Holy Spirit, who is a deposit guaranteeing our
inheritance until the redemption of those who are God’s possession to the
praise of His glory.”(Eph 1:13-14)
 The Holy Spirit is a
down payment, deposit, seal   this mean
that God has given us His Spirit guaranteeing that He is coming back for us.
Beloved, let us look at 
it from a natural perspective; If I  enter a store to buy  a jacket that costs $80 but  want to put 
the item  on layaway, first I will
have to  leave down payment  of $30 to secure my jacket, then I will be
given a time frame to complete my payment and take my jacket home; if someone
comes to the store and see my jacket and decides to purchase my jacket   even with the full amount  to pay for my jacket, that person would not
be able to take possession of it because my down payment has secured  my jacket 
until the time is expired. Likewise beloved God has given the down
payment of the Holy Spirit and He has time that only He knows, (Matthew 24:36)
that He will be returning ,so  within
that time the devil  can never  take possession of a believer because we are purchased  by the precious blood of Jesus.
“Now it is God who makes both us and you stand firm in Christ. He
anointed us, set His seal of ownership on us, and put His Spirit in our hearts
as a deposit, guaranteeing what is to come.”(2 Cor 1: 21-22)