We aren't talking about a man and wife (mere humans). We are talking about the Power of God to save and keep those who come to Him. Eternal life actually does mean ETERNAL, and any lack of trust in God and His ability to perform what He began in us means there is no justification...simply because it is lack of belief.
1 John 5:13
These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
1 John 5:20
And we know that the Son of God is come, and hath given us an understanding, that we may know him that is true, and we are in him that is true, even in his Son Jesus Christ. This is the true God, and eternal life.
We are KEPT by the power of God, we don't KEEP ourselves. This the the assurance only a true believer can have. We don't doubt...we believe there is a place reserved for us in heaven.
1 Peter 1:4-6
To an inheritance incorruptible, and undefiled, and that fadeth not away, reserved in heaven for you, Who are kept by the power of God through faith unto salvation ready to be revealed in the last time. Wherein ye greatly rejoice, though now for a season, if need be, ye are in heaviness through manifold temptations: